How to measure a saddle
Saddle seat sizes are are often mis-stated due a misunderstanding of what the manufacturer is stating as the size. The seat size stated by the manufacturer is a bare tree measurement which means no seat padding..just a bare tree so once a saddle is padded up for comfort the seat size will always vary by just a bit versus the tree size stated by the manufacturer.
For example when Circle Y says the saddle is a 16" Circle Y Park and Trail..they are talking about the measurement on a bare tree that does not include padding. Once padded your actual seat size will vary from .25" to 1" or more so be sure to ask for both measurements.
Manufacturers like Blue Ribbon are notorious for padding up a saddle so the tree may measure 16' from the base of the horn to the stitching but there is 1.5" in padding so your actual seat size is much less then your stated tree size. Here are some examples for your consideration.
You can see on these two saddles the one on the left is a Broken Horn custom show saddle and it measures to be a 17" tree but is padded to a 16" seat size. The one on the right is a Dale Chavez Show Saddle and Dale used very little padding. The tree size is about 16.25" to the stitching and right at about 16: to the seat padding.
Each saddle manufacturer is very different so be sure to ask for two measurements on every saddle seat, the first is the measurement from the base of the horn to the stitching around the cantle..this will give you the tree size, next you want the measurement from the base of the horn to the front side of the padding which is your actual seat size, both measurements are very important for rider comfort.