The 7700's contrast will definitely be better - inherently because of the HD2+ chip, but also because of some of the contrast enhancing technologies that have become popular (on machines like the Panasonic 700u, the PE7700, and others.
As to screen size, vs brightness - personal taste. Let's just say that in a dark room, you'll have plenty of horsepower. You can always dim the lamp down however...
I run my 8700+ on a 140" screen right now (waiting for a new Firehawk at 128",) and I am a bit underpowered for my taste, but pretty sure the 128" and 1.3 gain will make me happy. If you've got room, and seating where you want it (not too close...), then you certainly can go for a 92, 100, 106 or 110" without any real concern (dark room).
I think within that range of screen sizes, (and surfaces), it will come down to where you like to sit in a theater - are you an up close person, half way back, etc.
Since you already have a 4805, you should know what pleases you. Remember the pixels will be about as visible on the 7700 as they are on the 4805 - if you sit half the distance to the screen when watching the 7700.
YOu can always (as some folks do) get the projector - shine on the wall, and figure out what size is ideal for your purposes.
And if you are really adverse to the amount of gray where there should be black, you can always choose a High contrast gray surface - that might be your ticket if you go 80 - 92 inches.
The fun part, is that the 7700 is so far beyond the 4805 you will almost certainly love any combination you decide on. Have fun! -art