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Though i am quite new to fantasy (2-3 months) and only have a small collection of models (750 points roughly), I have a TERRIBLE habit of going all googley-eyed "cool" stuff over whats best for my army. Example: the New High elf Dragon;?;Y
The model is absolutely beautiful. Incredibly dynamic, impressive wingspan, Ect. With a few vines and spites, it could easily be turned into a forest dragon, which is even better. The thing is though, these new dragons are mounted on chariot bases instead of the regular 50x50 monster bases that our normal dragons are mounted on. I know that in 40k, the base is ok as long as it's size is equal to or greater than the base it is supposed to be mounted on. Is this the way it works in fantasy as well?
PS: to all those who say a larger base size is impractical, i know. I just NEED to find a way to get one of those killer dragons in a WE army.
GW used to release base size charts detailing exactly what size base everything was supposed to be put on, but a couple of releases have thrown the last version of it into confusion - specifically, the Giant and this new Dragon. The last base size chart released said that a Giant was supposed to be on a 50x50mm base, but the new Giant model (released with the Orc & Gobbo army book) came with a 50x100mm base (chariot size).
This, of course, raised some hackles because a larger base means more models that can fight the monster on a flank charge. While on a giant I question the logic behind allowing more models to attack the flank, on a dragon it makes sense. My friends and I have agreed that whatever base size the model comes with is probably the size it should have (within reason, of course - a box of dryads that mistakenly come with 20mm bases are clearly still meant to go on 25mm bases). This creates a fair amount of inequity, especially when you've got 2 giants in an army, one on 50x50mm and one on 50x100mm, but until GW releases something more official, I think it's the best we've got.
In short, the only harm in having the new dragon is it means more models can attack you on the dragon's flank. Since the book says monsters go on a "40mm or larger base" there's no reason you can't do it.
To my knowledge, the HE Dragon comes with a base. So, why are you wanting to make your own?
Oh well, onto the question, yes, I believe it is Chariot-sized. I have a TM, a Giant, and my opponent has a Carnosaur. *goes to bedroom to check sizes. The TM is 500x500, and the Giant is 500x750. I don't think size is terribly important. Just make it bigger than "Ogre-sized" so ti counts as a monstrous creature.