I = very small (obsolete by 1960)
J = small (started in 1950)
K = medium
L = medium (same size grip as K, but larger/heavier frame)
N = Large
X = Xtra Large (Mdls 460 and 500)
Z = the Governor
If you go to S&W's web page for revolvers Revolvers | Smith & Wesson there is a pull-down on the far right side for frame size, and you can use that to see what models (by number) fall into which category.
No, you are confusing serial numbering with the frame size. After WWII, the N frames were serialized with a serial starting with an "S". Around 1969 they changed the serial numbers to start with an "N" instead of the "S" and they continued that with the N frames until they changed over to their present serial numbering scheme of 3 letters and 4 numbers.