Hi dude.
Using this calculator from Optoma's site (
http://www.optoma.co.uk/distancecalc.aspx?PC=HD75) you can expect around a 90" image from about 10.5 feet. Awesome huh?
I can't comment on the 75HD, but I will be buying the 65HD shortly and I have read only positive things about it. With the 75HD being a higher spec model I would imagine it is just as awesome.
As for the light in the room. If you want to be able to see the screen well in a light room, I think the projector needs to have a higher Lumens rating. Someone with more know how can perhaps correct me on this.
The 75HD has 1400 lumens whilst the 65HD has 1600, so based on this I think the 65HD might be better in lighter conditions. It can be had for as little as £430 delivered which is why I am going for it
Good luck mate.