Denon/Marantz showed off a set of overhead in-ceiling speakers in their CEDIA demo this past fall that had 8" woofers and their other speakers maxed out at 6.5". With modern room correction, the woofer sizes aren't quite as critical, especially with a receiver that can handle bass management and frequency corrections like most current receivers.
Brand matching to me is still important as you are getting the same flat target response from each of the Axiom products, as you already know, but a slight variance in woofer size is fine. In fact, they will help to fill in a little of the deeper frequencies in Atmos playback even when the sound is noticeably coming from the traditional surround speakers.
