The correct method of measurement for an 'English' saddle is different from the method used for a 'Western' saddle. A Western saddle measures larger than an English saddle. For example:
a Western 15" saddle equates to a 16 1/2" or a 17" English saddle.

To get the correct measurement for an English saddle, measure a straight line from the saddle 'nail' (on the side in the front of the saddle) to the center of the cantle (the back of the saddle).

Generally, most sizes below 17" are for Junior riders or very small adults. The 17" and 17 1/2" saddles are the most popular sizes, with 18" and above being more suited to the larger rider.

The width of a saddle is usually denoted by one of the following letters:
  N (narrow),
  M (medium),
  W (wide), or
  XW (extra-wide).

To find the approximate width of an English saddle (in inches), measure from one side to the other, across the front of the saddle (the gullet), where the 'nail' is.

If you want more information, try this site:

Saddle Fitting Guide