Looking to see how large can I go with the picture and not loose too much quality.- Up to 300" according to the HD33 maual. (thats insane) It also depends on your room size. You could build a custom screen out of blackout cloth any size you like. I don't know your budget but this would be pretty cheap to do and they work great.
What are the disadvantages of shooting on the wall vs a screen.- Paint on screens are fine but if you want to hide your speakers, build a false wall and hang a screen on it. Put your speakers behind that.
If a screen is a must any suggestions , grey, white, gain etc??????-Again depends. How big is your room, how dark is your room? If you can get it super dark, go for a white screen. If you like your blacks Really black, go with a grey screen. Even if you do go the blackout cloth route, it is paintable as well. Just search blackout cloth screens on google for more ideas. Check out some screen size calculators as well....good luck!