I'm currently using a 7" and he's around 21/2yr old. So far I think it's working ok, it's just it looks small but there doesn't appear to be any pinching, marked by dry spots on his back after working.
We went from a 32.5 in an English saddle to a 36-37, I'd have to look again when he turned 4, considered an extra wide tree......... I was going to get a wide but the lady said that he would probably continue to grow and I'd be better off getting the wider tree so I wouldn't be buying a new saddle in a couple years and I'm so glad I did! If you get an English saddle, try to get one with wool flocking so it can be altered to fit him as he grows :)
I just bought a thorowgood cob saddle for my norwegian fjord pony after taking 13 saddles on trial periods and all failed the fit test. It's an all purpose english saddle synthetic with an XX WIDE gullet with FISH fitting system. She is short-backed, wide back with wide mutton wither. Everything pinched her shoulders or bridged her back from being too long on her short back. The only saddle that fit well was an old australian saddle my friend had (but won't sell it). When I save up enough I will have to get a custom australian saddle fitted to her. Western saddles just do not fit, not even the treeless ones. BTW the thorowgood is incredibly uncomfortable although I am used to my custom devoucoux super duper padded saddle so I am spoiled.
Our stallion fits a M/W - W (depending on the saddle... and how much work he's had). He has a fairly "narrow" frame for a Draft. Seems most comfortable in a SQHB if he goes western. Though he has a large shoulder - his back isn't that wide, nor is he particularly "stocky" for a draft horse. He is an 8yr old Clydesdale.
Our mare is much much wider. I actually couldn't find a treed saddle to fit her body type - she has very wide, large shoulders, with a very wide (double) back... after trying many different brands I decided to go treeless, and it works very well for her, it also allows for her changing shape and growth (she's still only 6 years old). Also a Clydesdale.
I found this one time in a bargain box at a tack shop and picked it up for $5.00 It has been a great addition to my tack room. I have a Percheron cross and a Haflinger. I used it on both of them to make a template of their withers which I took with me when I went saddle shopping. I slipped the template up into the saddle to find the right size for my horses, it worked great!