I have bits in shank sizes 6mm, 1/4", 3/8", 10mm, 12mm, and 1/2". And there are a lot more than that. Lots of CNC straight bits are the same size as the diameter and they use collets for each size, so if you are buying bits online it's a good idea to make sure what the shank size is.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.