The smallest lungfish species is Protopterus amphibius. Grows to about 18". But, good luck finding one. I had one over a decade ago. Have not even heard of another one available since then.
BTW, this is an african species. Meaning there's no such thing as mellow. African LFs have a tendency of ripping tankmates up.
The only LFs I've been able to comm are AULs and L. paradoxa.