Often the first question asked when someone wants to buy a catfish which grows to larger than average proportions is "what size tank do i need to keep it" and often when they get they reply they are shocked by just how large the aquarium needed to keep the said species needs to be.

The basic formula on tank to fish size ratio is 4x1.5x1.5:1 thats a tank of 4 times the fish length long, 1.5 times the fish length wide and 1.5 times the fish length tall. This provides the minimum ammount of space required for the fish to be able to swim and turn around freely without bumping into the glass continually.
*Note some highly active fish will need to have this taken to 5x2x1.5:1 while sedentry inactive fish can have it reduced to 3x1.5x1:1

This is fine for fish up to around 12 inches in length, a 12" fish only requires a tank of 48x18x18" so a standard 75 gallon tank is more than adequate but once you step over the threshold and enter the world of tankbusters the tank sizes increase dramatically, a fish of 2 feet long will require a tank of 96x36x36" (around 500 gallons) which is a big comitment to make to keeping fish and will probably require some structural modifications to your floor.

With big fish and big tanks comes the need for big filters and equipment, forget your aqua clears, bio wheels and fluval canisters, these are designed with average sized aquaria in mind and certainly dont have the media capacity to handle the solids that a 2 foot long catfish can leave. Now you have to start looking at custom built sump (wet/dry) filtration or the extra large range of Classic canisters built by Eheim, remember you want to turn over the entire volume of the tank around 5 times per hour so that 500 gallon tank is going to need a pump with a flow rate of at least 2500gph. Heating the tank will require 4x 350w heaters unless you keep your house at a steady 26c and if you intend to have live plants then you will need a set of metal halides blasting out 1500w of light 12 hours a day, this is going to draw a lot of power so expect your electricity bill to rise sharply.

If you have read all this and still think you have what it takes to keep a catfish which will grow monster proportions then i wish you luck and success and look forward to seeing photos of your monster set ups in the future.

what are you counting as a big cat. i used to think Limas were absolutely enormous but compared to many others theyre just not that big anymore

It doesn't seem to matter exactly what you consider to be big - the specified ratio should apply to pretty much anything if you also take activity into consideration.

I just bought a fish called an eelcatfish, I just found out that its actauly a walking catfish x_X they grow 1 foot long sometimes longer, also my gorami got madd at it and bit its tall off x_X its still eating and doing fine but... so far bad luck.