Are you talking about simulation or actual field measurements? My suggestion would be to contact the antenna manufacturer (or one that makes similar antennas) and ask them for a recommended procedure. They had to make the measurements in an OATS for something that size so you should be able to duplicate their setup without a lot of sophisticated equipment. A calibrate measurement antenna and a spectrum analyzer should let you do reasonable relative power measurements around the antenna field.
Here is a nice collection of satellite system calculators for you. They happened to be posted here on RF Cafe this morning (not by RF Cafe but a link to somebody's website). There's lots of great stuff there.http://www.satellite-calculations.com/