Meh. If he's going to make up non-RAW rules (which is, essentially, what he is doing here) that penalize you for the length of your weapon, I would have demanded at least reach for it. "Right. So I can't bring it into the bar cuz it's 15+ feet long. Okay, I get it. I can't hit the guy that's 10 feet away from me with my 15 foot greatsword because...?"
What does your char wear for gear? Full plate and shield? Do the patrons have any problems with the fact that your party is dressed to fight a minor war? Does the town? Does your GM know that running around in full heavy armour would be extremely fatiguing under almost all circumstances? That it was almost always restricted to planned, short engagements--or horsemen? Or how about the fact that swinging almost any killing weapon is extremely fatiguing? Have you travelled through a desert--geared up the entire time in full metal armour yet? And don't get me going on the whole 'realism' of the DND hit point system! : )
Let's go back to the greatsword, on a more serious note. We've established the GM won't let you take it into a tavern because it's too big. I would be seriously nervous about entering any dungeon. If the hall is 10' wide or less (heck, 20' wide or less!) is he suddenly going to invoke 'realism' again and refuse to let you wield it? Are you suddenly going to have to have no allies beside you to be able to properly swing at an enemy?
I always hate it when my fellow GMs suddenly limit a player due to 'realism'--in DND. DND is *not* a realistic game! It's about larger-than-life heroes swinging swords and wielding magical powers against horrible monsters/BBEGs.
Flavour text--and the length of a huge greatsword is flavour in DND, and nothing but--can be used to justify anything. And regardless of whether the GM's motives were outright malicious or not--they very well might not have been--he is, inarguably, cherrypicking his enforcement of 'realism'. --In other words, whether he intends it or not, he's employing favouritism and hypocrisy.
*shrug*