What type of seats will you be using?
What type of screen?
My room is 19' deep and eyeballs in the 1st row are 9-1/2' back from a 115" wide 2.35.1 screen. I wouldn't want to be any closer. I've also sat 9-1/2' from a 105" 16:9 screen, and that is pretty much how close I can be without feeling uncomfortable. With a 100" screen, you might be able to cheat your front row to 9', but that's really close. Depending on your screen type, you may start to see the weave in the screen at that distance.
Eyeballs in the 2nd row are a little over 16' back, which doesn't leave much room behind the 2nd row. When fully reclined (sleep mode), 2nd row heads will be very close to the back wall. When normally reclined during movie watching, I have a few feet of breathing room, but I would have preferred more to get the most out of the rear surrounds.
So, to answer your question, I wouldn't make the room any smaller than 19' if you plan on having 2 rows of reclining seats. In a shorter room, you might look at reclining seats in the front and traditional theater seats or a couch in the back.