Help! How to determine sizing for dog coat?

I bought this pattern to make a dog coat for my MIL's new jack russell terrier:

http://www.butterick.com/item/B5126.htm

Only problem is I'm not quite sure which size to make.  She lives in another state so I will not see the dog before the holidays and I can't ask her to measure the dog without raising suspicion.  I have seen pictures and the dog appears to be pretty standard for the breed (she looks like Eddie on Frasier) and I know she weighs 16 lbs.  I'm thinking I should go with either the small or medium.  The pattern looks fairly forgiving, but I am a cat person so it's hard to judge.  Anyone have any thoughts? Suggestions?

My son just moved back home and brought his two toy fox terriers with. I bought a few patterns and also found a really great one online free but haven't taken the time to make any yet. We saw an ad locally that a store was selling sweaters for only $1.99 each so we drove there and bought them before they were gone to use until I make more. On that packaging it lists the size and what dog that should fit. Sadly the small and XS sweaters were gone so we got a medium.

Anyway, the medium said it is 14" to 16" size and the dogs it is supposed to fit are Miniature poodle and Terriers: Boston, Fox, Scot, and Welsh. I can tell you that they are too long for both his dogs. The back goes way over their little stubby tails, which, as you can imagine isn't too nice considering that area kinda needs to be clear for them to potty. HA We've had to fold those back at least four inches to keep the length away from their tail areas. On of his dogs weigh about 5 lbs and the other about 10 lbs. Because of that I think you should probably make the size Small for the dog you are looking to make this for.

Another idea is to make the small and also the medium. Then the one that doesn't fit, ask your friend to donate that to a local pet charity to use there.

I think the medium would definitely be too big as it would fit my miniature beagle, who is definitely larger than a jack russell. Plus, I think it's better for the coat to be a little short than a little long. A long coat could really get in the way of the dog doing his business.