This has got to be the fifth or sixth harness we try and so far (knock on wood, fingers crossed, toss a black cat over your left shoulder...) it seems to fit him and not cause him any issues.
The story so far: We started with a no-pull harness ("easy walker") which never really fit him well: The medium was great around the girth but too tight around the chest (which lead to rubbing in his armpits), the large one was too loose around the girth but fit well around his chest. I managed to make it a bit tighter around the chest, so that worked for a while. But the harness overall is pretty stiff and was still rubbing his armpits (especially when he stops in mid-walk because he (once again!) smelled something really, really interesting!). So he ended up with some fur loss in that area.
That's when we tried another one that was essentially a collar with two additional straps around the legs. Worked so-so.
So we decided to use them until he had learned how to walk nicely on the leash (most of the time) and then switched back to the collar. And what a difference that made - he definitely enjoyed the walks a lot more. And as he was not pulling (unless there was another dog) and only every once in a while stopped or jumped to the side I figured we'd be ok.
Well, he developed a cough/gag which our vet suspects is because of a slight inflammation of his trachea... due to the collar. So - back to the harness to take stress off his trachea.
That was about a week ago and since then we've tried a good 3-4 different harnesses. I found one which seemed to work very well, until he became more and more scared of it. No wonder, it had rubbed his armpits and tummy quite badly. No idea how that could have happened, it fit pretty well (I had that double checked by the seller). Maybe it happened in the dog park? Who knows.
So I tried padding that harness some more. Not much luck.
Also briefly tried the halti, which I think is a great idea but doggie is not too fond of having something around his mouth (yes, we need to work on that!). So long term that might be a solution. Short term - no.
I finally found this harness which is mostly soft and flexible material. So far he seems to be doing ok in it. Of course, he is still a bit hesitant when he sees it, and walking down the stairs in it is still a challenge (mental, of course. Silly puppy.), but I haven't seen any new chafing or rubbing. Fingers crossed. If this doesn't work ...