The Xfce4-settings-manager allows a maximum workspace margin of 262 pixels (?).
I think, that's to small for the horizontal margin. In the past i used about 400 which allowed to place a xterm of convenient width beside the normal windows.
Created attachment 2179
Fix max margin values
Nice catch, actually the max margins are inverted for horizontal and vertical.
Please apply the attached patch.
Applied, revision 29513.