I have a teacup and longhaired Chihuahua. The teacup would be able to wear the XXS harnes but the chest for the longhaired would be to big as well as the neck. The neck wouldn't be so bad. It's the chest size that is always larger in Chihuahuas. Hope this helps you as a guide.
Yes they are. But only so much. As well as the neck. I just won an XS here and it fit my Teacup but not the longhair. Some breeds have very large chests. My Peke was a nightmare. I LOVE these harnesses and would use no other. MANY times I've had to yank my dog quickly up into my arms becasue of a loose snapping bigger dog. I can't bend down that well and these have be terriffic!
LOL My dog is like that with a sweater. The long haired. She goes into a deep freeze and will actually fall over if you touch her. Stiff as a board! LOL These are MUCH better than a reg collar. A collar can either slip over their head when you have to pull them up quickly or even choke them. THESE are the BEST! Unfortunately, they're not cheap to buy. Getting them here is MUCH better. lol Good luck on your auction. HIGH BIDS!