They certainly go extra wide, I'm not sure if it matters what size they started at though, I seem to remember someone saying they'll easily go 2 sizes larger from their start. Julia at PMG knows a bit about them, I think Barrie Swain told her last time that her's couldn't go any wider

I've got one, it fits Rija's back shape but is currently too narrow because she had it fitted in the winter and is now carrying summer weight :

It might fit again soon but I'll need to check.
As I know you know, to get any treed saddle to fit they will need checking and re-adjusting every time the horse changes shape and if your boy is building muscle it might not be practical for you to have your own saddle when you aren't there all the time? Some pros have their saddles refitted every 2 weeks!!!
St.Merryns are fantastic saddles though, a good choice
