If you've got enough breakers available, do 3, it's easy to do when you're already there, rather than deciding to do it later.
A 108" viewing distance divided by a 90" screen equals 1.2 - MAX, you are correct!
Likewise 108" divided by 1.2 = 90" (for the peanut gallery)
Do you already have the couch? If not, I'd get the biggest one you can, you'll always have seating, and there's a comfortable amount of walking room on either side - I wish I had that! Plus, you'll have less than 3 feet on either side of your screen, great place for artwork, sculptures can be natural diffusers, for example.
Yes, larger speakers can typically play lower frequencies better, but high and mid range drivers need very little space to operate. Check out Dennis Murphy's BMR Monitor
http://www.philharmonicaudio.com for an example of a 3-way speaker that can anchor the front end of a multichannel system and then be quite capable even with out a subwoofer for 2 channel music listening. Dennis is on this forum and I can speak for his work!
My contention is, in that size space, more satisfaction will be derived from higher quality speakers, rather than a higher quantity of speakers. I would start there, and then as time and money allows add more. ATMOS is only the beginning, the new 16 channel processors are already way down in price