If it was based off a Brachiosaur it would be larger and longer than it is already, I have to agree that it was based off of an Apatosaurus. Not to mention This is my favorite pokemon and I very much dislike it's portrayal as a turtwig.
Apatosaurus was 75 feet long and brachiosarus was only 85 feet long, not that big of a difference. Apatosaurus did not hold its head up like meganium either, which was clearly a reference to old brachiosaur paleoart.
I think the design was more based on the body shape rather than full out size, though looking at it more i think The Pokemon would have been more of a diplodocus. Though I think the design was probably supposed to mimic just long neck dinosaurs in general and they just picked out a dinosaur name.