Welcome. An 80 cm Dish should be fine , most people use 90cm to be safe. 1.2 m is an overkill. I beleive AMC 4 has RTPi , if that is the case even the smaller 60 cm is fine since RTPi signal is so strong (although on very sunny days you get adjacent interference from the adjacent satellites). It also has un-encrypted , History E and BIO East channels as well.

In my opinion this website which is exclusively dedicated to Satellite matters has more info , SatelliteGuys.US .