you can't unless the developer has scripted it
Quoting the Chaos Project Forums, user ForeverZer0 says:
It is not really that difficult to change the Game.exe to allow for resizeable windows, that's not really the problem. The problem is with the all the built in RMXP scripts that made to use a 640x480 resolution. You can actually change the window size with a script that isn't more 10 lines long, but will show only blackness around a 640x480 picture.
I wrote the resolution script so that a new resolution could be used and have things re-center themselves correctly and draw the tilemap, weather, viewports, etc. to fill the window. I could have easilt included a way for the window size to be changed in game with a script call, but omitted it due to the fact that it would be highly inefficient and prone to errors to re-draw and configure for a different window size once the game is already running. This would be 1000 times worse if the window was resizeable using the mouse. If I were to re-built the RMXP engine from scratch, it would not be difficult to include the feature and have it work correctly, but is really not practical with the current system.
effectively RPG Maker XP's window size can't be changed unless it's been scripted which apparently requires altering all the classes, and infact Fullscreen Mode is still 640 x 480 which is why everything looks blurry in Fullscreen
as such in general with RPG Maker Games there's no way to do it. this would have to be developed by the developer themselves. without that all you can do is what you have done with a short cut or
- use Alt + Enter to switch between Fullscreen and Window Mode
- lower your screen resolution closer to 640 x 480 (RPG Maker XP's window size)