The Dolby guys (and gals) plus at least 2 of the other speaker demo companies were all using relatively small speakers for their overhead speakers. Now granted, the SteinwayLyndorf demo used some advanced bass management, or so they said, so that the smaller Atmos speakers sounded like full range speakers. Not sure how the "average" Atmos receiver or processor will handle that, but they sounded like full range speakers, or at least a lot more than just high tweeters, in all of the demos.

None of the overhead speakers were more than a single tweeter and single 5.25" or around that size. M3s with their slightly larger "woofer" would probably meld better with the Axiom fronts than the M2s, but I don't think that it is as critical based off of what I saw and heard. I mean most of the front speakers were massive beasts compared to the smaller overheads, and they still sounded well balanced.

I would, however, stay away from something like the M22s for overheads as they have the tweeter and dual drivers.
2-M60s, VP180, 8-M3s, SVS 20-39PCi, DIY Sub, 8-Shakers, JVC RS45, Anthem MRX-1120