I would not fret about it too much.
Yes, you will have some of the issues people talk about. As has been explained already the larger the screen is and the closer you sit there will be a degradation in picture quality as your eye is able to see more and more detail. Here is a chart to calculate viewing angles http://myhometheater.homestead.com/viewingdistancecalculator.html
It looks like your viewing angle is around 34 degrees which is not very large compared to most around here with projectors but is still much much larger than what most people experience in their homes with a standard TV.
I have experimented a little bit with sizes of images and viewing distances with my 360 and if you go too big you will notice the picture quality degrading. But, you can still go huge and get a great looking picture. How big might be a matter of taste and the brightness level you end up with will make a difference in the overall picture quality as well. Make sure your room is pitch black and painted in dark flat colors of course.
I would just do some experimenting yourself and once you come up with a size you like just use it. Does your projector have lens memory? This would make it super easy. You will want to some up with some kind of masking system if you will not be using the entire screen to the borders of the black velvet frame.
And like others pointed out the Xbox One is just 2 months away so resolution will be very low on the list of picture quality issues. Your projector and screen and room itself will probably play the biggest part in picture quality at that point.