1280x1024 is more normal than 1200x1024 for rdesktop.
You can specify any display resolution you want; it doesn't have to be one of the "normal" ones. And you can switch back and forth by just starting a new rdesktop and "grabbing" the rlogin session away from the previous one. But once an rdesktop is started up, its display resolution is fixed. If you make it smaller than the previous one, it will push your windows around to fit.