The minimum size for a tank for an adult or a juvenile is 40 gallons. You should definitly not keep a 14 inch bearded dragon in a 20 gallon tank, as those are best for babies. It could be bad because your bearded dragon is very active at the age he probably is at 14 inches, and he needs more space to move around. If he doesn't have enough space, he could become stressed and unhappy. If he didn't have enough room to run he could get unhealthy too.
A thirty gallon would be better but he'd grow fast, so 40 gallon or higher would be best. Good luck.