That depends partly on how many receivers you plan on hooking up to your dish. Figure how many you have (or may have in the future) and get a 4xwhatever number of receivers, probably a 4x4 at a minimum.
Most people nowadays seem to go with a 4x8 or 5x8(includes input for antenna) for futureproofing. This is a fairly popular MS;
http://www.solidsignal.com/prod_display.asp?PROD=SPBMS58
If you are in an area getting locals on one of the new satellite allocations (72.5W), a 6x8 might be best.
http://www.solidsignal.com/prod_display.asp?main_cat=02&CAT=&PROD=SAM-6802
If nothing else, call SolidSignal and get a recommendation.