You must design your forms for the resolution at which they will be run. If your users are using 800x600 then you need to design your forms for 800x600. The form designer can assist you in this by setting it maximum design area to 800x600, however that setting does not account for the start bar at the bottom of the screen so you need to limit the height of your forms even more.

Also, once you redsign the forms for 800x600 they will still be too large for 640x480. You have to decide what the target resolution for your system is and design to that resolution.