Looking over various sword websites, the longest blade length I can find is for a Zweihander with a blade length of about 48" and an overall length of 65". Most Claymores I've seen online are about 39-40", with regular one handed swords being 29"-32".
I'd say that the size increase would be proportional - a large greatsword could be anywhere from 25% larger to 100% larger, depending on if the creature it's made for is at the low end of "large" or the high end. Divide that 48" blade by a 6' tall man and you get 8" of blade per foot of height. Add about 3" per foot of height for the grip, and you should be in the ballpark.