Whenever you connect to a remote server via RDP, you will automatically get the last session’s resolution. When your own RDP resolution is higher than the last one, then the new RDP session resolution will increase to match your own resolution.
However, when the last RDP session was created with a higher resolution, then you will end up with scrollbars around the RDP window.
You can resolve this in Windows 7 with a feature called RDP smart sizing.
If you created an .RDP file for a specific server, then you can edit that one. You can also edit the default RDP file so every session to a new server will take this setting.
The default location for the default.rdp file is %USERPROFILE%\Documents\default.rdp.
Add the next line to the end of the default.rdp file.
Now your RDP session will take the resolution you want and not the highest one since the last reboot of the server.