Actually, I'll do one better. Our parent company Immersaview just installed a racing center in Brisbane. The pictures show what two W1080ST's are capable of and general idea on mounting and screen arc configuration. The screen size can be smaller, just use a smaller radius.
http://www.immersaview.com/showcase/mxj-f1-simulatorNthusim Plus can achieve that same setup. Immersaview Warp uses the same core as Nthusim Plus. Make note that the projectors are given sufficient space around them for ventilation and exhaust. Also make note this is the approximately 120 degrees a dual setup could easily do. With 120 degrees in mind look at the location where the racer might sit vs the edge and depth of the screen. The view is still forward facing and you do not have a maximized FOV. I think it would be really cool with 21:9 Blu-Ray content though while not gaming. The screen size is at the extreme range that I'd run 1080p at. That is a pretty tall projection height. Again, with consumer home builds I wouldn't go over 60". The racing setup shows the extreme you can go to in these sort of builds though.