How can I run Plant vs Zombies in a resizable window mode?

You could use software such as ResizeEnable to make the window resizable, although I can't vouch for the quality of this specific product, having not used it.

If there's a settings.ini file or something similar somewhere in your PvZ folder, you might be able to change the default size of the windowed application as well.

Plants vs Zombies has a fixed resolution of 800*600, so you won't be able to do this unfortunately

It's a bit overboard, but you could do like the accepted answer on this question: Is it possible to run RollerCoaster Tycoon 2 in windowed mode?

You could install Virtual PC on Windows 7, install and run PvZ on that, and then size it the way you want.

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.