This policy setting allows you to specify the maximum display resolution that can be used by each monitor used to display a Remote Desktop Services session. Limiting the resolution used to display a remote session can improve connection performance particularly over slow links and reduce server load.If you enable this policy setting you must specify a resolution width and height. The resolution specified will be the maximum resolution that can be used by each monitor used to display a Remote Desktop Services session.If you disable or do not configure this policy setting the maximum resolution that can be used by each monitor to display a Remote Desktop Services session will be determined by the values specified on the Display Settings tab in the Remote Desktop Session Host Configuration tool.
Windows Components\Remote Desktop Services\Remote Desktop Session Host\Remote Session Environment
Machine
At least Windows Server 2008 R2 or Windows 7
HKLM\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services!MaxXResolution HKLM\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services!MaxYResolution
TerminalServer.admx