Capacity is important, but footprint (dimensions) are also very useful in determining good choices.
Do you want a solitary fish, or shoal of? You could easily get away with, say, a dozen Pimelodus pictus, or even half a dozen Sorubim lima - tank furniture would be important for the latter to ensure they had enough places to hide if needed. Alternatively, how about 6 Ornate Pims (Pimelodus ornatus) Not sure what 'pink convicts' are, and if they are aggressive - I'm guessing you should be OK in a large tank for them not to get too territorial, and equally, if you wanted to go for a large solitary catfish that would not outgrow your tank, say something like Aguarunichthys torosus (bolt catfish), you'd need to be sure there was nothing that would become a snack.
We need more info on what you want from a catfish - large, predatory, or smaller, looks good in a shoal, etc.
Martin