What kind of harness helps a dog that pulls?

Short answer: one where the lead clips on at the chest. With a back clip, your dog leans into the harness and pulls you along like a sled. With a front clip, every pull turns them slightly sideways, so pulling stops getting them anywhere.

It helps, it doesn't train

Most people feel a difference on the first walk. But a harness only makes pulling less rewarding. If you want a dog that walks nicely on a loose lead, you still need to practise: stop when they pull, move again when the lead goes slack. Boring, but it works.

Measuring

Wrap a soft tape around the widest part of the chest, just behind the front legs. Go by that number, not your dog's weight. If they fall between two sizes, pick the bigger one and tighten the straps.

Checking the fit

You should be able to slide two fingers under any strap. If it rubs under the front legs after a walk, loosen the chest strap a little. If your dog can back out of it, it's too loose.