X,N,L,K,J (in modern S&W's) Big to little, although L and K can be a little difficult to tell apart until you have compared a few of them.
If you want to get even more detailed there is also J Magnum, Improved I, I and M frames out there. And real picky would include the G (stainless N), H (Stainless L), F (Stainless K) and E (Stainless J).
There also is a whole list of two and three letter designations that point to more details. i.e. NAT = Aluminum N frame with Target sights, etc.