Well on a 17' wide room you would be looking at a huge screen which would then require a light cannon so the prices start rising fast on the whole project. I'd say you would be looking right around an 11' -12' wide screen and probably closer to 12'.
My room is 18' wide and I had to have a 13' wide screen to fit all of them behind it. And mine are closer to the edge than I would like but they are back there now.
If I were you I wouldnt really worry about having them behind the screen unless you a worried about how the sound will be different between the L/R and the center. If you have EQ in line the center should be able to be matched to the L/R that are on the outside of the screen. Once you start getting into the really wide screens the prices just start going through the roof for both screens and projectors if you want a great picture.