I'm no expert by any means, but your space is close in size to mine. My HT room is 18.5' l × 10.5' w × 7.5' h. I also have 2 rows of 3 seats with the back row on a set of 16" risers I built.
Your room is a little wider, mine has more headroom. I'm currently running a 7.2 set up, with an acoustically transparent 110" screen. I also want to add 4 Atmos speakers, and another set of bed layer speakers for the front row.
IMO width wise, you've got more than enough room. If I had that extra 1.5' of width, my room would be near perfect. Your biggest issue will be headroom. Having the bed layer speakers at the ear height of the listeners, and then having Atmos speakers as well, which is pretty much the same issue as I'll be having when the time comes. On top of that, depending on your seating height, seeing over the front row could be an issue. The 16" risers are about the perfect height in my room. You said length wise you've got room to play, so that could definitely help out.
Are you planning on having a projector or just a big TV?