I've just bought a 16h 7y/o TB gelding and needing to purchase a saddle. I've seen one online but not sure what size to get, is there a general size that would fit this breed that i should get.
Definitely only use a qualified saddle fitter - different tree shapes from different makes fit different horses ans the fitter will be able to try a range to see what fits best and also watch you ride to ensure the saddle remains still and balanced when ridden in. There are so many problems a badly fitting saddle can cause it is imperative to get it right! My horse had an awful time getting a saddle to fit.
The inch size refers to you rather than the horse as it it important the saddle fits you also
Hope this helps and hope you find something
There are two sizes to every saddle - the width of the tree arch at the front and the length of the seat (measured from the centre of the branded nail head on the side of the pommel, through the air, to the centre of the cantle).
You can get an idea of the width of tree arch that the horse can take by making wither tracings (plenty of info on the web if you search) but seat length is more tricky and more variable. In the range of saddles I fit the length of the panel, the stuffed bit that sits directly on the horse, can vary by at least 1/2" on different models.
What you CAN do is find the last rib, which saddle fitters should show customers how to do. If you go to the soft part of the flank and come forwards you should be able to find the last rib, then track it upwards and forwards, and then follow that line once it goes under the large muscle that carries the saddle. The weight bearing part of the rear panel should sit on but not beyond this muscle. If you fit longer the horse can get very sore as there is no skeleton to support the weight of the rider.
So, the maxiumum seat size is determined by the horse, but that saddle may be too large or too small for the rider as an ideal. You can definitely go smaller for the rider, but you can't go larger - what you can do is look at various options on bench made saddles like an extended cantle which on a flat seated saddle can give you more room, or perhaps a shorter panel, though many makers will only put the matching size of panel on the tree.