I think it's been assumed (and therefore established as an informal standard) that MitD's mental age is equivalent to a human that is somewhere from 11-14, being either somewhat mature or extremely childish depending on which end of that spectrum, but childish nonetheless.
Originally Posted by CriticalFailure
While it sounds easy to project a child going from one age group to the next being like one size category to the next, you would only see a change in any size category at all from newborn to adult (assuming a species is not specifically on the cusp of a certain size). Most animals have significant changes in growth, but going from near maturity (say, a preteen) to an adult would not usually mean a size category change unless they are already on the border.
Think of it in human terms: while humans go through significant growth spurts around puberty, the growth spurts aren't nearly as big as a size category difference. We might grow over 20% of the original height (from a preteen to a fully grown person), but the change from one size category to the next is double the size. A shift in size that significant is like instead of your 11 year old cousin going from 5'3'' to 6'2'' by the time he is 18, he inexplicably goes from 5'3'' to 9'8''. This just gets worse as you go higher, with going from Large to Colossal being tantamount to that 5'3'' kid growing to 40 feet. (For reference, from Large to Gargantuan would be 20 feet. Still implausible.)
So if MitD is a very big Large now, and the adult size is unknown, he could plausibly turn out to be a moderate Huge with some margin of error. But if MitD is a very small Huge now (or even anything that is not a very big Huge), he cannot turn out to be a Gargantuan when he is an adult based on "normal" animal growth.
A reference that illustrates this visually:
Spoiler: Spoilered for size
MitD is most likely somewhere between the knight and the good-sized ogre. Colossal, which is not shown here, is over double the size of Gargantuan.
Another example of this, courtesy of Grey Wolf
Originally Posted by Grey_Wolf_c