Solved! Yamaha TSR 5830.

[JJake]

what are the best speaker sizes to install in a ceiling with this unit. 8ohms 6.5". watts? what is your pick for Yamaha TSRm5830

 

[americanaudiophile]

In this case the amp isn't the determining factor. The size of the room and your application is.
Atmos speakers can be small since the Atmos channels have no bass. Even 4" woofers will be OK. Wide dispersion is a good thing in this application.
https://www.cambridgeaudio.com/usa/en/products/speakers/c46
If you want to do in-ceiling front/surrounds you can't do Atmos. The non Atmos speakers have to be in wall and mounted much lower for Atmos to work correctly.
There are in-ceiling speakers that fire at an angle rather than straight down. These work well are the L/C/R speakers and can also be used in other applications if the speakers can't be placed where they should be.
https://www.paradigm.com/en/in-ceiling-speakers/ci-home-h65-a
Whether you have a subwoofer affects whether an 8" woofer is important.
High ceilings mean the speakers will be further away from you so will need to play louder.