WOW, thanks for the response. I gave up on this thread days ago!
The problem turned out to be a setting in the ATI Hydravision Desktop Manager, where you can have the program "automatically align monitors", but I think that's used in the case where both displays are the same physical size, so I unchecked the box and the problem was solved. I ended up back at 800 x 600 anyway because text and stuff was too small for my old eyes.
As far as frequency, I assume you're talking about the refresh rate? Although I don't think that would have any effect on screen size, I keep it at 75 Hz.
AT