What size frame is right for me? I have an inseam of around 26 1/2 inches (without shoes). Would a 47cm frame fit me? If not what would the correct frame size be? I'm looking for a hybrid (or road) bike.
Really, the only way to make sure you get a good fit is to sit on and test ride some bikes. There are many variables that sizing guides can't take into account, such as proportions of the rider's arms legs and torsos, and preferred riding position. Additionally, the common seat tube measurement isn't very standardized across manufacturers and frame styles 

Getting the right size bike is worth putting some effort into. Presumably you want an bike you can enjoy riding, You won't enjoy riding an incorrectly sized bike. Even if you can manage one visit to a good bike shop to try a couple bikes would probably be worth it. 

All of that said, if you really need to buy online, you definitely need at least a 'small' size. Look for manufacturer's size guides of bikes you are considering and for good return policies in case your estimate is off.