If it helps at all, historically, Bastard Sword were pretty much the same size as what would have been called a Greatsword (It is a term fairly vague that could design many type of oversized sword). Zweihander and other huge renaissance era two-handed sword were never called Greatsword AFAIK.
But then again, a Longsword was the same size as a Bastard Sword too.
I know, RL doesn't help D&D much. The only point I'm making is that it's all in our perception. What we think a greatsword should be. And 4e shifted on us the Greatsword and Bastard Sword. But more often than not, that image we have in our head about what weapons are like wouldn't even exist without D&D.
If you try out Riddle of Steel, a realistical and more historically accurate game, you are in for a shock. ''So that's a Longsword?! What, it's not effective when used with a shield!? My head!''