Lots of nice, smallish border collie mixes around in shelters I bet My girl is a jack russell x border collie, she weighs 26lbs, but most of the terrier/BC mixes I know are a lot smaller than her.
As someone else said, you can find nice small border collies too. A friend of mine has a working bred BC girl that is the same size as my terrier/BC! I find that BCs tend to get annoyed with Elsie though, they don't seem to enjoy her play style that much.
If you're looking for actual breeds, I find miniature poodles tend to get on well with BCs, they are SUPER snuggly, will keep up well on walks and playing chase with BCs and are very clever too.