Mitsubishi HC6800 Projector Screen Recommendations
Since the Mitsubishi HC6800's black level performance is good, but not up to the better, ultra-high contrast projectors, and since I'm huge on black level performance, I would tend to recommend a high contrast grey surface to lower black levels a bit.
I've mostly been watching the HC6800 on my Firehawk G3. I've done some watching, filling the whole screen, but have done the bulk of my viewing reducing the screen size to about 112" diagonal, where thanks to all those "best" mode lumens, the Mitsubishi projector manages a fair amount of pop, at the smaller size.
Trying to do a larger size, such as my 128" grey, should work fine, initially. Remember though, that those lamps dim over time, and my bet, is that by the time the lamp hits, perhaps 1500 of its 2000 hour rating at full power, the HC6800 will be getting dim on the Firehawk.
If you aren't all that concerned about black levels, and don't have side ambient light that a high contrast grey screen surface can help with, you have lots of options.
If you do want to go large - and you are primarily focused on movie viewing, a modest gain white surface will give you more brightness and punch to the image. Filling just about all of my Carada Brilliant White screen with 1.4 gain, the 106" diagonal image was bright! And vibrant! Nice.
Add it all up, and thanks to the almost 600 best mode lumens, you've got a lot of options to match a screen to this projector and your viewing requirements.