Fifteen feet deep should be enough for 2 rows of seating, one row against the back wall if needed.
Put things that you might put at the back on the sides instead.
With just one really wide row of perhaps 8 to 10 seats the sight lines become too unfavorable (although this could justify habitually stretching 4:3 shows to fill a 16:9 screen to give persons on the ends a better view).
If a passageway needs to run along one edge of a square room you might give a little thought to make that edge one side instead of the back.
Remember when lines and logos burned the TV screen? I was at a concert where a musical selection made extremely heavy use of about four of the keys of the piano.