Yup, small diameter RTAs are rare at any power level. Probably more out there that take heads, which are mostly rebuildable too. Lately I've been looking for wider and shorter low power RTAs for better pocketability and there are few of those too.
I think there is GG gear that is small in diameter. If you don't mind clones they've probably been done. I have a
Vision Eternity that is tiny enough to fit on a eGo battery. Performance isn't fantastic, but is surprisingly good for a tank that size.
If you don't mind dripping there are some really great tiny RDAs out there too.
Good luck with it