When designing and developing screens for our desktop application GiftWorks we design to a window size of 800x600. Oddly enough there isn't an easy way to do this in Windows. I've used some utilities before but have recently stumbled upon a very simepl utlity for doing just that.
Sizer sits quietly in your taskbar tray and subtly modifies every window with options to set the precise size of a window. The specific sizes can be easily configured if the defaults aren't adequate. I really like this program. It's simple and focuses on a single function.
A product of Brian Apps