[Saulot]

There is no size 7. Size 6 is an abstraction for all creatures larger than size 5, even if relatively they are not quite similar in size with each other... like a tarrasque and an elephant.

The main obstacle with bigger creatures is not the size limitation, but the grid size on the battlefield, and the 128x128 pixel limit. Even if the pixel limit was removed, larger sprites would begin to overlap (in fact, they can even at sizes within the 128 limit already) due to grid spacing.

Edit: For those of you thinking, well.. maybe in Dom4 we can ask for size12 grids which are about 4 times the size of the current ones (and have 6 size2 units), that comes with it's own baggage, namely that you're going to have units which attack other units with melee weapons that couldn't possibly reach them.