I used 4" cans in my room. I very much like them in a low ceiling (7'10") but in the rest of my home where I have 9' ceilings I used 6" cans.
The 4" cans just looked right to me for my theater. I based my comment of you not having enough light based on my room. My room is only 10'2" x 20'6" approx. and I have 6 lights in just the ceiling and it's really marginal for having enough light. Plus, I prefer to run cans somewhat dimmed as it doesn't seem as harsh as running them full on. With that in mind, I like to use more cans and use just dim them to the preferred lighting.
My little theater has 23 cans in it. 14 of them are in the soffits and 3 above the stage and they were meant to be accent lighting, but with as dark as the room is, they are also needed for the times that I need the room brighter, such as for cleaning.
For your room, I'm would probably put 5 across the back, 5 across the middle and 5 (or 4 if the stairs are in the way) across the front. I would want at least 2 zones and possibly 3, one for each row.
There is not any right or wrong answer, it's all what you want.