Intermedia followed by kelberi.
Kelberi will get 24" and so will Orino's.
Who knows...Intermedia may get large as well in aquariums, they haven't been readily available until now.
I like to think in terms of slowest growing compared to smallest growing because IMO they all have the potential to reach atleast 20" with proper care and space. All three of the species mentioned are usually the slowest growing species.