My maximum resolution is 1280x768, and the water properties window (in the highest position possible) still runs off the bottom of the screen with my taskbar hidden. I can't resize the windows in GECK, there's no drag handles so it's impossible to click the Ok button. Hitting Enter doesn't work for water properties though so I'm a bit stuck. Any ideas besides buying a new monitor?

Go to "User/YourUsername/Documents/MyGames/Fallout3" and open GeckPrefs.ini and in that look around for settings that have to do with the windows you need to resize. Then enter a smaller value, like 200-500 so they are smaller. Then resize them yourself in the geck to the appropriate size.

Therein lies the problem, there's no setting in there that controls the size of the "Water Type" window. Can I add it somehow?

Then just delete the GECKprefs.ini and when you next open the geck it should detect a default setting smaller than your resolution and fix it.

Thanks for trying to help, but deleting Geckprefs.ini didn't solve the problem.

Water Type properties is the only window that's causing me trouble. Every other window fits on my screen and responds to "Enter" as Ok, but not that one. It still runs off the bottom of the screen so I think I'll just leave water alone until I get a high res monitor.

Do you have any resolutions where the '768' is bigger?
If you do then try it.