A 20 inch catfish would weigh around 2.5 to 3 pounds, in that area anyways.
A good rule of thumb is most fish that are around 30 inches in length would weigh about 10 pounds. Thats true with walleye, catfish, redfish, etc.
Unless you get into fish that are skinnier like pike and trout, they would weigh a little less.
Fish that are even wider as they get bigger, like black drum, freshwater drum, largemouth bass and smallmouth bass, carp, etc, they would be even on the heavier side of weight.
But you can figure it out from there, so a channel catfish around 24 inches was probably 5.5 pounds.