theres loads, the shark catfish (pangius?) gets huge , Tiger shovel nose can get a few feet long as can the red tail catfish. There was a documentry about one of them not so long ago think it was somthing gigas?
when you say tropical i guess you mean freshwater as sharks are probally the biggest marine predator,One of the largest freshwater fish in the world is the Arapaima, Arapaima gigas, then as said again you have redtailed catfish and tigershovel noses and channa marulia-channa micropeltes-