Bikes – and especially bike frame – are often referenced by a size, like 56cm or 27 inches. For those “in the know” this is a good approximation to indicate if a particular bike will generally fit you. While this may not be all that critical when you are in a bike shop and can actually throw a leg over the bike, this number can be important when you are looking on Craig’s list, for example. So what does this number actually mean??