I used to have a 16 inch and it fit me ok
I went to Robinsons on Saturday to try some but they only had an 18 inch in. I have a feeling I have sat in a 14 inch before.
I took the stuffed cantle out of my 17 inch close contact and put the solid cantle under the seat at 14 inches and managed to get my bum in it ok so it might be viable.