Personally. I use a tape measure and I measure “dot to dot”-- or between those tufts of wool at the top of the panels at the front of the gullet. If those tufts are not there, I still measure from those “corners” of the panel. The tape measure lets you get right in there with your hands and stretch the tape between those narrow points. IME, a stiff tape measure will yield a less accurate measurement unless it’s done by a skilled and honest seller.
A Medium tree of a yestercentury Crosby will be 4" and maybe a tad. I would think of a wide as 4.75" or greater. It will be hard to find wider than that.
I agree with others-- measuring seat size in English saddles involves a standard technique. It should not be hard to figure out the size, though moving the nail can make it vary by a quarter inch or so.
Also, no, no, no to replacing foam panels with wool. I think y’all have covered this, but I have seen a saddle ruined this way. Even with new panels made, they looked fluffy and the panel got really narrow… narrower than the narrow of the original Crosby saddles. It was unusable.
I do agree with your love of those Crosby Equilibrium saddles-- for both horse and rider. The panels on those and the tree shape made them very horse-friendly, even if they are narrow by modern standards.
I think most Crosbys did have the seat size stamped on the sweat flap, but it would not surprise me if that were gone by now.