Before you go and buy a second hand english saddle look up www.sustainabledressage.com
-in the tack section they discuss saddles - everything you want to know about shape, manufacture, fitting etc.
Unless we are talking about a pony it is unlikely that for a horse the saddle should be less than 17inch - the length is for the size of your butt not the horses,
What also is important is the width, ie narrow, standard, wide or even extra wide for cobbie broad backed horses.
Because of the smaller footprint it is extremely important that the saddle fits the shape of the horses back. Practically speaking you should not buy a saddle that has not been first fitted on the horses back to check that it fits length ways and width ways.
Then there is the question of type ie general purpose or dressage or jumping.
But you should sit on that saddle also to make sure you feel comfortable.
Read the website and then you'll understand the principles.
If you buy the wrong sized saddle, the horse will be uncomfotable and will develop evasions.
In Britain we send off a form supplied by the maker to show the shape of the horses back together with a trace of the back of the horse from side to side at certain key spots - . Key criteria are
type ie general purpose, Length ie 17,5 inch,, width ie wide
The manufacturers will send the saddle and if it doesn't suit, you can send it back.
Alternatively you go to the local tack shop and they deliver the saddle to your stable , you place it on the horse, you ride around and if it does feel right you try another.
But to judge whether a saddle fits properly or not takes a skilled eye.
PS I have 5 English cut saddles - all of them fit my butt but only one fits my horses back.