A few years ago I probably would have said that it doesn't matter but as surround systems and recordings evolve I am learning that you want to have good speakers in every position, not just the front 3. Size is not the only determining factor, though. You want a neutral speaker (even frequency response) that has a good frequency response both on and off axis, so you need to look at characteristics of each speaker. The 6.2 inch will likely have a lower frequency response, but if you have a subwoofer and cross at 80Hz then it's not so important whether one speaker goes to 70Hz and the other 60Hz. Get the best speaker that fits your budget. The room will often restrict what size speakers can be used, but that's ok as we all have to make some compromises.
Yes, there is no restriction that says ATMOS has to be an in-ceiling or in-wall speaker. Most choose in-ceiling for aesthetics as a box speaker tends to look odd mounted on the ceiling. What is important for ATMOS is proper placement. Go on the Dolby website and look for the
Dolby specifications for speaker placement. The one thing we do not recommend are the up-firing ATMOS speakers that bounce sound off of the ceiling. On ceiling works much better.
