When I was screen sharing today, using Finder, I found I could not get it to show me the full 23" screen from the remote computer on my 21" monitor. It worked fine before, but today it was messed up. No matter which mode I chose it was larger than my window.

All I had to do was delete com.apple.ARDAgent.plist from my root Library preferences folder and all is well!

[robg adds: I don't have this file (in either Preferences folder), even though I use the Finder's screen sharing feature regularly. I'm guessing that it will only be present if you've used the full Apple Remote Desktop package.]