Depends upon the application. If it's a primary application - that is, if you expect many users to use primarily this application for their work and seldom use another, I think it's OK to expect them to use 800x600 (especially if it runs on Win95 or NT, or if it's for accounting-type tabbed data entry - you know, heads up keyboard only stuff when you don't want the user looking for the mouse).

If it's more of an incidental application, supporting their primary work in other applications, keep the window small (even if you ARE expecting an 800x600 desktop). Check out the book 'About Face' for some other excellent guidlines along these lines.

If it's for older Windows clients, I'd try to stick with 640x480 in either case. I've gone through this kind of thing - if you need 800x600 to show everything, you're probably putting too much on your form! Try to break it up into logical tabbed pages.

FWIW...