It is possible to set two different resolutions for your Parallels Workstation VM - one for windowed mode and one for full-screen mode. When in window mode I like to have a smaller screen so that I can more easily navigate to my other Mac applications. While in full screen mode, I like to maximize the screen real estate available.
Luckily Parallels Workstation enables me to set a different screen resolution for each mode, except they made it hard to find, and not very intuitive (maybe it is in the user guide, but who reads manuals?).
In order to allow for different screen resolutions for window vs. full-screen mode you must go to Preferences/User Interface and uncheck "Change Mac OS X resolution". Now go back to your VM and set a screen resolution while in window mode (1024x768), switch to full screen and set a different resolution (1440x900). You can now switch back and forth between window vs. full-screen and preserve the different screen resolutions for each one!