The collars that Barb and I got are not adjustable so measuring accurately is very important. If Smudge is still growing, I would probably hold off until he's done. Callie's rolled collar adjusts down to 14" and it slips over her head so that was one of the measurements I sent to them. Her LBD martingale slips over her head easily and once it's on, she can't back out of it.
There are adjustable with buckles martingale collars available but they are much wider- they may only come in regular flat collar style- I didn't spend much time with that type since I was looking at the rolled style leather and once Barb took the plunge to see how it worked, then I still waited until I was sure that I would actually use it.