Don't go too big. I wear a 10.5 shoe, and 26 boots. They're the right length for my foot. However, going too tight isn't necessarily a good thing either. I had bad fitting boots last season (not the length, but the instep, toebox, and heel of the boot were fitted bad to my foot), and they cut off my circulation, and I lost 2 toenails.
Moral of the story: get fitted by someone who really knows what they're talking about. Fit tight, but not painful. I thought painfully tight was good last season, and my feet were in pain to the point where skiing wasn't as much fun.