OP, after doing a little research, I think what they had may have actually been Hydrocynus vittatus and not the Goliath variety, but it still might be worth checking out.
This is one of those questions that you'll get 10 different responses if you ask 10 different people. For a true Hydrocynus goliath (GATF), I would not keep a full grown specimen in anything under a 12'L x 12'W x 3'H (3,000G) tank/pond. They grow up to 5' and weigh 100lbs+...but they are nothing like a huge catfish that slowly swims around the bottom of the tank...these fish are lightening quick, open water swimmers.