The Great Goblin is also much bigger than I had imagined him myself.
Truth is, though, that the exact size was not given. Surely he was a lot bigger than a regular Goblin.
In several places in his works, Tolkien writes about Orcs being smaller than Men. Goblin is mostly used as a synonym for Orcs. Uruks were about the same size as Men, and very rarely some Orc Chieftains were said to be Man-size too (like the one that stabbed Frodo near Balin's Tomb).
Now when we apply all we know to say something about the Great Goblin's size, I think it would be most logical to assume that he was about man-size or a little larger (when standing up). So, personally I'm thinking about 7' at the very most (like the highest people of Middle-earth, the Numenoreans). Now he was probably of great girth (I think there's some quotes on that). His exceptionally length (for an Orc/Goblin) and belly -I reckon- are why he received his name.
By the way, I just remembered this:
Quote:
Suddenly a sword flashed in its own light. Bilbo saw it go right through the Great Goblin as he stood dumbfounded in the middle of his rage. He fell dead, and the goblin soldiers fled before the sword shrieking into the darkness.
So there we have it. Glamdring went right through the Great Goblin. With the Great Goblin being the size as it is now, I don't see this happening! I guess we can safely assume he was meant to be smaller.
@Lord Hurin: in some published letters from the last years of Tolkien it's revealed that he thought that some Maiar had perhaps taken the form of Orcs too. He then speculates that the Great Goblin might have been a Boldog too.
Now when the Hobbit was written, Tolkien did most likely not yet think of him as a Boldog. In the last few years before he died, he re-wrote a lot of stuff, re-thinking lots of things. The Boldog is first mentioned in this time, many long years after the release of the Hobbit.