A small is 12-13" from the base of the neck to the base of the tail. a x-tra small would be 9-11" from the base of neck to the base of the tail. i would say a small with your chi. mainly because it wraps around the upper body and clips on top. a lil loose won't hurt nor will it come off, now if you put a tea cup chi it would probably swallow her or him. i see no problem with the 5 pounder. hope this helps..thanks for asking. it's hard to read the pictures so they are no help there.
My sister gave 2 to me. so i'm not sure where it came from. you would need a x-tra small. it has a norfolk terrier on the small size one. measure the base of the neck to the base of the tail and if measures 12-13" it may fit her, might be a lil loose, but you can only try. thanks for asking.
this is the same harness i have for my little dog.....and its great, he stays a little warmer, it doesnt rub on his skin, and i put it on over his sweater in the winter as an extra layer......by far my favorite, and he has had several