is it a 50 gallon breeder tank or a 50 gallon fish tank??? There is a big difference. The breeder tank will have more floor space which is definantly a must for a dragon whereas the fish tank will be taller than it is wider which really doesnt work well.
Depends on the measurements. Most standard 50/55g tanks are 4 feet long but only 12 inches wide, so they're both too narrow and provide insufficient floor space (only 4 sq. ft. instead of the recommended 6-8 minimum) for an adult dragon. Some use a 40g breeder tank (3 ft long x 18 inches wide) as their minimum adult tank size, but IMO this is less than ideal since it also doesn't meet the floor space requirements. The smallest size I could recommend for an adult dragon would be a 75g tank (4 ft long x 18 inches wide) - it meets the minimums for both width and floor space.
I have a regular 55 gallon aquarium (for my frogs) and I too think it isn't wide enough. They look huge, but Oscar is about 18" in length and the tank is only 12" in width. I would think he would feel a bit squished when turning around.
I have seen the breeder size tanks and think they are wide enough. Many people use a 40gallon breeder tank for adult dragons.
I use a 40 gal. breeder (sorry, panicked a while ago because I'm terribly OCD when it comes to my beardie), and it seems to work just fine. When I bought it, the man I got it from said it was 50 gal. If you're buying your tank second hand, or even right from the store, bring an accurate tape measure, just in case, and measure the dimensions. While I'm sure some people won't look upon this too kindly because it looks like you're questioning their judgement, it is better to be safe than sorry and waste all that money (and if you buy it second-hand, there usually are no returns). Sometimes the people selling them don't know what they're talking about. Again, this depends how big your beardie will get within the next few months. I think anything under 18" wide is just too cramped up.
I have a 50 gal reptile tank (not an aquarium), and that one works just fine. Yes, I have two in there. Before people start biting my head off, I let them out to run around my apartment quite frequently so they get their exercise and satisfy their insane curiosities.