Go to a local bike store. Different manufacturers have different ways they measure size and you can only discover this by talking to someone who knows what they are selling. For example I just bought a new BMC in size 58. I also tried a Pinarello...the shop knew that they are sized different and I needed a 56 in a Pinarello. It felt the same as the 58 BMC. Some manufacturers like Giant don't use numerical sizes at all but use S, M, L etc instead. Get in there and have a professional sizing done to know what you need.