I've had 6 foot RFMs and one 7' RFM.
My opinion is the 6' unit is the better choice. It's not just a matter of the tractor's PTO HP....I ran a 6' with a 24 PTO Deere with no problems. To me, the size (width) of the RFM is more dependent on what your lawn is like. If your lawn is flat as a pancake (no bumps and depressions), a larger unit would do. If the lawn has those liitle bumps and depressions, a larger RFM may scalp or leave grass uncut due to the width (spanning those lower areas rather then cutting).
Another thing to consider...when turning, an RFM's caster wheels may dig in, especially with softer soil. The bigger the RFM, the more weight and likelihood of rutting when turning.
All that said, I prefer a ZTR with a 60" deck. Faster, easier on the lawn.