How do I size a choke chain collar?

I don't like the finer links myself. Generally, the chains are of a weight reasonable to the length. You're not going to find a chain heavy enough for a mastiff sized for a cocker spaniel. You are probably only going to have a few choices in weight for the proper length of chain.

You said terrier, so the dog is probably more muscular for his size than some of the lap-dog types. I'd go with medium-to-heavy FOR HIS SIZE (not the heavier gauges for larger dogs of course).

That's just me though.

I like to find a chain that slides through itself easily and when you run it through your hands, it should feel smooth. Some manufacturers make slightly squared-off links that feel sharper to your hands. The better quality ones (probably more expensive) will be smoother and heavier (if it feels really solid and heavy, you might want the medium weighted ones). LOL, now that I'm thinking of different manufacturers, this question is harder to answer than I first thought. ;)

If you buy a cheap one, get the heaviest probably. If you buy a better one, get the middle weight. ;)

use a harness instead of a choke chain

you take to store and have trainer fit it for you

i use harness on small breeds for walking and training