Okay, I would say if your up to 5'6 get a small, up to 5'9 or 5'10 get a medium, about this get a large, or if your fooking massive, get an XL.
As for sizes 14 & 16 are small, 17 & 18 are medium, 19 & 20 are large & above are XL,
I am 5'8 & I feel comfortable on a 17' or my current is a 17.5 which is a medium.