How To measure the seat size on an english saddle

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Measure your saddle from the front "nail head" on the pommel, to the very (center of the) back of the seat to get the proper seat measurement, as shown at left. If you normally ride in a 15" Western saddle, then you should ride in an English seat two inches larger, usually a 17" seat.

A 16" Western Saddle is equivalent to a 18" English Saddle  
        English          Western             Australian  
    15"               13"                   15"
    16"               14"                   16"
    17"               15"                   17"
    18"               16"                   18"
    19"               17"                   19"

Tree Size & Gullet Width

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The gullet of a saddle is the channel that runs down the center of the underside of a saddle, in between the panels. The gullet bridges the horse’s spine so it is an important part of the saddle design. If the gullet is too narrow, it can pinch the muscles on either side of the horse's spine which ultimately will make it's back very stiff or sore.

Why do manufacturers make these really narrow gullets?
Many people like riding in a saddle with a narrow “twist”, and some saddle manufacturers simply bring the panels closer together toward the back of the saddle to accomplish this. However, it’s not nearly so comfortable for your horse!

Gullet Width
On your saddle is important too.
Narrow Gullet
Wide Gullet
Many people confuse Tree Size with Gullet Width. The tree is a completely different measurement: it refers to the distance between the bottom of the points of the tree. This measurement usually described by riders as narrow, medium, medium/wide, extra wide etc.
Can a gullet be too wide? Yes, if it reduces the size of the panels to the point where they can no longer adequately distribute the rider’s weight properly. This however, is a fairly unusual; it is more typical for gullets to be too narrow.