Well it depends on what you're doing with it. The government recommendation for portions of vegetable is that 80g is one portion - this is about the size of a fist. If you follow that then you're looking at 16kg.
I feed my catfish macaroni, frozen peas should be fine for a catfish. If the fish cant swallow something, generally it cant fit it in its mouth. Plus, they swim in their own feces, vegetables won't harm it.