I would go w/ something at least 24" wide, and probably 8 feet long. With that you will also have room for maybe some sunfish or a bullhead catfish. The bass will need that much width so it can turn around once it get bigger.

Even a 14lbs northern lm is rare. I wouldnt expect it to gorw more than 8lbs in an aquarium, with a length of about 24." Any lm over 10lbs is considered a trophy by most fishermen, and in states w/ the northern strain, a 5 pounder is a trophy.
"he" is actually a "she"