Treeless saddles............?

Hi, I'd like to get a treeless saddle for both my Cob and youngster to share preferably, as my youngster is growing rapidly, and my Cob changes shape often throughout the year, no matter how hard I try to keep him trim. I've heard they adapt to every horse's shape, but I've also seen they sell in for instance 15 inch, 17 inch etc. So how would I measure my boys for it?