This seems a little bit confusing, as far as I know, 2.55 was the original name of the style because it came out on Feb. 1955. However, because chanel released the reissue last year (reissue is a term we use here only, the SA has no idea what reissue is), the SA calls the reissue the "2.55," so the other classic flaps, usually made of lambskin or caviar, the SA calls those "classic flap". If you say 2.55, the SA would think you are talking about the reissue. But in general the style itself is 2.55.
224, 225, 226, 227 and 228 are part of (i believe is the last 3 digits) the model numbers of the last year reissue, they are specific sizes for the reissues only. 224 is the smallest and 228 is the biggest. The size 225 is the size of the original design when it came out back in 1955.
With the "classic flap," which is usually made of lambskin or caviar, the sizes are usually refered to as small, medium and jumbo I think.