How big can you print? Well I have a good friend who has just had a 4 megapixel photo printed onto a billboard so I think you'll be fine. When you print larger, you stand further back, nobody looks at an A1 print under a microscope
So at this size you only need to print at 100 dpi for example, Ill let you do the maths.
I have recently been using a website called photobox (I think dot com, but google it) for any large work or anything I care about.
I think they have a great selection of papers/inks/qualities etc as well as very high quality from my experience. I recently had a 30 by 20 inch printed from them with a 12 megapixel camera, and thats come out great, even if you stand right next to it.