There's a whole bunch of factors like seating distance, screen ratio, mounting location, ceiling height, preferred content, etc... My personal suggestion would be to go as large as you can stand. There's a reason you're getting front projection instead of a 80" TV! I'd see how large the image is before I start noticing the pixel structure of the projected image. Then watch it for a while to make sure it works. I do here of people being initially overwhelmed by the size and later wishing they got a larger screen after getting accustomed to the size.
There's all sorts of math you can do based on THX guidelines, Throw ratio, seating distance calculators, and on and on. As suggested previously, there's no substitute for just playing around with the image on a wall if it's feasible. You'll need to make sure the size works for you in your room with your equipment (speakers and such). Then you also have to consider how you want to handle Cinemascope movies versus standard HD and whether you tolerate black bars or not. You're going to create a million more questions for yourself as you start deciding on a screen and size. Have fun!