I have a gargantuan mini (the 3e Black) and a collossal mini (Reaper's Kaladrax). It takes years of play before the PCs can get to fight these. They make good unique end bosses but I don't need lots of generic gargantuan & collossal dragon stats.
Likewise I have a couple horse-sized dragons (Reaper's young white/red, and the WoTC 3e baby black) that just about fit a medium base, but I have little need for medium dragon stats - these are really all closer to Large.
Conversely I have tons & tons of Large & Huge dragon minis, that's definitely where the action is. So I suggest probably 2 tiers of Large stats & 2 tiers of Huge stats might be most useful. Add a tier of small/medium wyrmling stats if desired.
Sizing dragons (or really any serpentine creature) has always been difficult for me.
If you look at typical dragon images, a dragon can be divided into three sections: neck, body, and tail. The neck of a dragon is usually either as long as the body or slightly shorter. The tail of the dragon is usually either as long as the body or slightly longer.
Measurements given for a dragon are usually from the snout to tip of tail. So a 30' long dragon can be thought of as roughly 10' of neck, 10' of body, and 10' of tail. That's a big creature, but how big is this creature?
The horse is generally thought of as an iconic large creature, while the elephant is generally considered an iconic huge creature. A typical male elephant is about 18' long. The dragon, while longer, is mostly serpentine. If you line the dragon up against the elephant, the first 10' are mostly thin neck, then you have some body, and then the part that sticks out past the elephant is mostly thin tail. If the elephant sticks it's trunk out, it's nearly as long as the dragon. Perhaps more surprisingly, the shoulders of this 30' long dragon are actually only 4-5' high, with another couple of feet rising above that to anchor the long neck. Although the neck allows the dragon to rear up to a great height, it's body isn't really much taller than a person. The elephant is thicker, taller, and more heavily built. The bull elephant (about 12000 lb) weighs much more than the 30' long dragon (about 5000 lb).
That's more than twice the size of a Clydesdale, so ok, this 30' long dragon is Huge. But it's also clear that it can get a lot bigger before we ever need to consider Gargantuan. If you look at the size table though, 30' is near the top end for a huge creature. But 5000lb is near the bottom end. The top end on weight is 16 tons, which a dragon won't reach until it's about 55' feat long.
The top end on gargantuan is an astounding 125 tons. A dragon doesn't reach that until its 110' long! So Colossal dragons are BIG! Right now, none of the dragons on my table come remotely close to Colossal Size - nor do any of the dragons in the 1e Monster Manual. Similar serpentine creatures like a Brontosaurus, often treated as the iconic Colossal creature, are also not Colossal by these estimates. Indeed, the Brontosaurus is more serpentine than a typical dragon, with a thinner body, neck, and tail. Despite hitting 72' long, they only top out at about 16 tones - barely into the Gargantuan range. To get to Colossal scale, a sauropod has to hit like 140' long, with a body scale of 4x10 squares and a reach of 40'.
Snakes are even more extreme. A 170' long snake, even one with a heavy body plan, is only solidly in the Gargantuan range (77 tons) in terms of weight. It can curl up in that 20'x20' gargantuan space (assuming you use 3.5 bases), but it's reach is over 50'.
All that is mostly just musing, except to note that the biggest dragon currently on my 1e table scales in 3e only to Gargantuan, and only reds, blues, and a rare green hit that size. Likewise, while the top end of 3e reds is a colossal dragon that is like CR 26 (and they deliberately understated dragon CR to make them 'scarier'), where as the top end 1e dragon (huge red) only scales to about CR 16 on my table assuming a faithful conversion to 3e. (And if I cared about 4e, presumably scales to about 24th level in 4e.)
So to get back to where this thread came from, to fill out anything above those sizes involves going from the 10 size categories of late 1e (based on innovations in dragon articles) to the 12 size categories of 2e, and even then probably wouldn't involve anything with as many as 40HD.
So another way to look at this is, do we need those very high end colossal scale epic dragons? I don't feel like I do, and I'd rather get 'worse than the worst' by way of templates and unique dragons, but what do others think?