I don't know if people actually measure? Most standard to smallish horses are 12 inch front, 14 inch back. Horses with much taller cannons are 14 inches, and 16 inches. I use 12/14 on my large pony and my 16.1 fine legged TB. I use 14/16 on my 17.2 WB (who has slightly finer legs) and my 16.3 WB with big bone.
Most shipping pillow wraps are too short - especially for our 16++ hand ponies. But - to answer your question - take a tape measure to the barn. Drop the line from right at/below the knee to the ground. (Since shipping wraps should tuck below the heels). Alternatively, do the same with a piece of bailing twine, bring it home to measure the length.
I don't wrap for shipping, just for stalls. I've seen people do the MEGALONG wraps for shipping but more commonly I've seen more along the lines of the length I use for wrapping in the stall and standing wraps.
The one time I shipped in wraps (I typically use shipping boots), I did bell boots too and had the wraps been longer, that wouldn't have been possible.