My solution for 2020 year, Windows 10.
(AMD Radeon required)

1. In the Radeon drivers settings enable the ReLive. Run the game in windowed mode (alt+enter) because ReLive seems to be unable to record in 65k color depth. There enable the Region Recording Hotkey and Borderless Region Capture. Enable Show Overlay in the main settings tab. Then when you use the Region Recording Hotkey it shows the overlay where you chose the Red Alert 2 application and it starts recording. Weird thing is that the recorded file has your screen resolution even while the window was smaller.

2. Since modern monitors have much higher DPI game details are too small and hard to see so I use the Windows Magnifier. In the RA2.ini settings file I set the resolution as 1/2 of my native monitor resolution:

    [Video]
    ScreenWidth=1280
    ScreenHeight=720
   

Now move the game window to the corner of the screen with Win+Right, Win+Down keys. It breaks the size of the window so I Alt+Tab to unstick the mouse from the game and drag the left top corner of the game window to restore the correct window size. This procedure positions the window right in the bottom right corner so now when you move the mouse there and press Win+"+" several times it zooms perfectly to the borders of the game window. Also disable the "follow mouse" in the Magnifier settings.

This all also might need the CnC DirectDraw patch to be unpacked into the game folder.

So unlike the previous solution I don't need to record audio and video separately. The modern hardware accelerated recording technology easily records games in full resolution and fps.

But the solution is still a bit complex. The issue I might face is that Steam and Windows Notifications appear in that corner so maybe for the game I'll have to move the Windows Taskbar from the left border of the screen to the right one and place the game at the bottom left but I'm not sure the notifications won't switch the corner too. Needs testing. I've asked the question about Magnifier at Super User.

My solution for 2020 year, Windows 10.
(AMD Radeon specific)

1. In the Radeon drivers settings enable the ReLive. Run the game in windowed mode (alt+enter) because ReLive seems to be unable to record in 65k color depth (or because it's DirectDraw). There enable the Region Recording Hotkey and Borderless Region Capture. Enable Show Overlay in the main settings tab. Then when you use the Region Recording Hotkey it shows the overlay where you chose the Red Alert 2 application and it starts recording. Weird thing is that the recorded file has your screen resolution even while the window was smaller.

2. Since modern monitors have much higher DPI game details are too small and hard to see so I use the Windows Magnifier. In the RA2.ini settings file I set the resolution as 1/2 of my native monitor resolution:

    [Video]
    ScreenWidth=1280
    ScreenHeight=720
   

Now move the game window to the corner of the screen with Win+Right, Win+Down keys. It breaks the size of the window so I Alt+Tab to unstick the mouse from the game and drag the left top corner of the game window to restore the correct window size. This procedure positions the window right in the bottom right corner so now when you move the mouse there and press Win+"+" several times it zooms perfectly to the borders of the game window. Also disable the "follow mouse" in the Magnifier settings.

This all also might need the CnC DirectDraw patch to be unpacked into the game folder.

So unlike the previous solution I don't need to record audio and video separately. The modern hardware accelerated recording technology easily records games in full resolution and fps.

But the solution is still a bit complex. The issue I might face is that Steam and Windows Notifications appear in that corner so maybe for the game I'll have to move the Windows Taskbar from the left border of the screen to the right one and place the game at the bottom left but I'm not sure the notifications won't switch the corner too. Needs testing. I've asked the question about Magnifier at Super User.

UPD: new Radeon drivers have much complex interface and you might need to search for all these options using the search field. My approach is that the Windows taskbar is on the right, the game window is in the left bottom corner and instead of recording the specified window I use to record the full desktop after zooming with Magnifier while holding the mouse in the bottom left corner -- it takes just a second or two to start the process, not as difficult as you can imagine from looking at the size of this answer. If you prefer to start recording before you zoom in (if you want to see the Radeon drivers notification that the recording has been started) then just use Avidemux after that to cut the first seconds from the video (use the "Copy" options on Video and Audio streams so it will not reencode but just cut the seconds out).

My solution for 2020 year, Windows 10.
(AMD Radeon required)

1. In the Radeon drivers settings enable the ReLive. Run the game in windowed mode (alt+enter) because ReLive seems to be unable to record in 65k color depth. There enable the Region Recording Hotkey and Borderless Region Capture. Enable Show Overlay in the main settings tab. Then when you use the Region Recording Hotkey it shows the overlay where you chose the Red Alert 2 application and it starts recording. Weird thing is that the recorded file has your screen resolution even while the window was smaller.

2. Since modern monitors have much higher DPI game details are too small and hard to see so I use the Windows Magnifier. In the RA2.ini settings file I set the resolution as 1/2 of my native monitor resolution:

    [Video]
    ScreenWidth=1280
    ScreenHeight=720
   

Now move the game window to the corner of the screen with Win+Right, Win+Down keys. It breaks the size of the window so I Alt+Tab to unstick the mouse from the game and drag the left top corner of the game window to restore the correct window size. This procedure positions the window right in the bottom right corner so now when you move the mouse there and press Win+"+" several times it zooms perfectly to the borders of the game window. Also disable the "follow mouse" in the Magnifier settings.

So unlike the previous solution I don't need to record audio and video separately. The modern hardware accelerated recording technology easily records games in full resolution and fps.

But the solution is still a bit complex. The issue I might face is that Steam and Windows Notifications appear in that corner so maybe for the game I'll have to move the Windows Taskbar from the left border of the screen to the right one and place the game at the bottom left but I'm not sure the notifications won't switch the corner too. Needs testing. I've asked the question about Magnifier at Super User.

My solution for 2020 year, Windows 10.
(AMD Radeon required)

1. In the Radeon drivers settings enable the ReLive. Run the game in windowed mode (alt+enter) because ReLive seems to be unable to record in 65k color depth. There enable the Region Recording Hotkey and Borderless Region Capture. Enable Show Overlay in the main settings tab. Then when you use the Region Recording Hotkey it shows the overlay where you chose the Red Alert 2 application and it starts recording. Weird thing is that the recorded file has your screen resolution even while the window was smaller.

2. Since modern monitors have much higher DPI game details are too small and hard to see so I use the Windows Magnifier. In the RA2.ini settings file I set the resolution as 1/2 of my native monitor resolution:

    [Video]
    ScreenWidth=1280
    ScreenHeight=720
   

Now move the game window to the corner of the screen with Win+Right, Win+Down keys. It breaks the size of the window so I Alt+Tab to unstick the mouse from the game and drag the left top corner of the game window to restore the correct window size. This procedure positions the window right in the bottom right corner so now when you move the mouse there and press Win+"+" several times it zooms perfectly to the borders of the game window. Also disable the "follow mouse" in the Magnifier settings.

This all also might need the CnC DirectDraw patch to be unpacked into the game folder.

So unlike the previous solution I don't need to record audio and video separately. The modern hardware accelerated recording technology easily records games in full resolution and fps.

But the solution is still a bit complex. The issue I might face is that Steam and Windows Notifications appear in that corner so maybe for the game I'll have to move the Windows Taskbar from the left border of the screen to the right one and place the game at the bottom left but I'm not sure the notifications won't switch the corner too. Needs testing. I've asked the question about Magnifier at Super User.