You could push screen size little more if judging with a brand new lamp, but remember, these projector lamps all dim over time, and you would likely feel a little starved for lumens near the end of the lamp's life (4000 hours in this case). My recommendations provide a little buffer, but not enough to assure full satisfaction at the end of lamp life. Remember, it's subjective to a point!
The Epson has a ton (over 2.5 times the lumens) of extra brightness available in the three bright modes that do not use their "theater" filter: Natural, LivingRoom, and Dynamic, so bright modes with ambient light are not going to be your limiting factor.
I tend to favor HC gray surfaces unless you have a nice, perfect room (dark walls, floor, ceiling). If you have the ideal room, however, definitely consider the slight gain white surfaces, such as the industry standard Stewart Studiotek 130, the Carada Brilliant White surface, etc.
The HC gray surface will keep the blacks "blacker", but then, the Epson is already excellent at this. The bigger benefit of the HC gray surfaces, is rejecting unwanted ambient light, especially from the sides - covered windows and doors, sconces, etc.
For Sports viewing, I was able to even have one of my french door windows partially open when viewing in Dynamic mode, and a open a little, even in LivingRoom mode. Of course, I wouldn't have been able to get away with as much ambient light if my screen was a Studiotek 130 or equivalent, which would have washed out more.
A killer long term solution is a 100 or 106 inch screen with an HC gray surface, 110", though, is definitely doable, even with light walls. With darker walls/ceiling/floor (not necessarily black - say medium brown, you can definitely go up one screen size, say 115 - 118".
If you do want a screen the size of mine (128" diagonal) or larger, you will need to go with a true high-gain screen 1.8 gain or upward, or give up on the "best" modes and use LivingRoom, etc.
So, size aside for the moment; light walls - I favor the HC Gray. If you have dark walls with full lighting control, you can definitely go white surface (1.0 - 1.4 gain).
The Epson Home Cinema 6500 UB and its sibling, the Pro Cinema 7500 UB, are the brightest of the "high end" 3LCD home theater projectors out there.