I've had the same problem with the screen size limitation with the microsoft version of wm rdc. I'd say it is almost useless. I've since switched to Mochasoft's Remote Client:
http://www.mochasoft.dk/rd.htm . It uses a resolution of 800x600 and has 3 levels of zoom: level one is a compress version of the entire screen, level 2 is zoomed in a bit (can barely make out words), and level 3 is a 1:1 version of the remote system. In levels 2 or 3 there is a pan features that allows you to pan around on the remote screen.