Define upside-down catfish. There are multiple Synodontids sold under the same common name. I have Brachsynodontis batensoda, the giant inverting catfish, which is quite large and is almost always upside down, but it is not "the" upside-down catfish, which is Synodontis nigroventris - and it only get 4" or a tad over.
Go to Planet Catfish for more info and pictures