Depends on the projector. I would recommend you hang out at projectorcentral.com they have a calculator for each projector that will tell you what size screen you can get at any given distance.

My projector is 13ft back and I also sit about 13ft back (front row). My screen is either 105" diag for 16:9, or 130" diag for 2.35 material.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85