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.