My theater is a little smaller than yours about 12' x 14'. I have a 91" (16x9) screen and 1 row of seats.
Basement Theater
If you can, set up your projector and use folding chairs to size your screen and seating to your preferences.
If you can change the size of your room, another consideration is the acoustics. Some room dimensions are better than other dimensions. I was surprised how much a few inches make. Here is a link to a spreadsheet that I used.
Frequency Response Calculator This also helps with seating locations.
Keep us posted on your progress, Good luck!

Dave