You measure like a western saddle, to the back of the pommel.
Treeless saddles do not fit any horse. They will fit a wider variety of horses, but they are not a one-size-fits-all solution.
With treeless saddles, your pad is as important as your saddle. It's very important to get a saddle pad that is specifically made for treeless saddles. You need the pad to create stability and weight distribution. I have two treeless saddles and I use Skito pads with them. It works very nicely.
Also, do not look twice at cheap treeless saddles. They are dangerous and can be uncomfortable for the horse. Stay away from saddles made by Maya, Hilason, Sydney Saddleworks, and any "no-name" treeless saddle. These are all low quality Asian made saddles.
Good treeless brands include Bob Marshall, Circle Y, Barefoot, Sensation, Freeform, Bandos (runs SMALL!), Ghost, Torsion, Heather Moffett, Ansur, Black Forest, TreeFree, StarTrekk, MacKinder, Kuda, Rebecca Softrider, and Ez Fit. I have a Bob Marshall and a Black Forest.