Thanks so much for the tips!
Peter, my case was exactly like tphaggerty's. The contractor said "Unless there's a specific need for 2", we'll be doing 1 1/2" pipe on the return end." So I was just sizing up that need. It looks like 1 1/2" should be ok, since we'll have at most 6-8 panels with good sun. 2" pipe may be overkill for our size pool/panels, but might come in handy if we choose to divert return water to a water feature (a growing interest as we re-design things!), which would also run during daylight hours.
I think we need to take some time to digest this info here & all else we've received so far & go back to the design table for a few days. This is really helpful, so thanks for taking the time!