Plants Vs. Zombies' Windowed Mode runs the game at 800x600. You can access the windowed game by unchecking the 'Fullscreen' option in the menu here:
You mentioned that 800x600 is too small for your screen. As a workaround that doesn't require installing additional software, you could temporarily lower your PC's screen resolution.
On Windows 7 and up:
- Right-Click the Desktop
- Click Screen Resolution
- Lower your screen resolution and click Apply.
You can also adjust the size of the taskbar to be smaller if you find this takes too much space:
- Right-Click the Taskbar->Properties
- Check the 'Use Small Icons' box
- Apply and OK
This wont make the window resizable, but will make it larger on screen relative to the resolution that the OS is outputting.