I have had this happen to me quite a few times when using a Remote Desktop connection. The RDP connection screen would go funny and then refuse to maximise again. So I decided to do a write up to save me time in the future.
The solution is really quite simple. Go to Start --> Programs --> Accessories. Launch Remote Desktop by clicking on the icon. Then click on the Options button. You should then get the following screen come up.
Select the Display tab and set the Remote Desktop Size to Full Screen (by moving the slider fully to the right).
If you want to have several remote desktop connections of varying sizes you can create shortcuts to the as follows:
Shortcut name:%SystemRoot%\system32\mstsc.exe /w:1600 /h:1200