Okay, so, someone tell me if I have this right.
1) Egg Size in the raws doesn't seem to determine hatch time. The Child tag controls the hatching.
1a) I would think that, if egg size DID control hatching time, not having the child tag would simply mean that the dragon would hatch as an adult, but be of the size determined also in the raws. Oh well.
2) On body size. According to the raws, all dragons, until they reach 1000 years old, are of size 6000. At year 1000, according to the raw, their size then changes to 25 million. Wow!
In all the fortresses I've ever encountered a dragon, I don't think it was ever 1000 years or older. I've never generated a world that was that old, in fact all my generated worlds are generated only up to 125 years. Now, I've never really looked at the legends stuff until very recently (it's a whole new dimension of fun for me), so I don't KNOW for certain how old those few dragons might have been.
But in my current game, the dragon that appeared at my fortress that I trapped (invader dragons aren't trap immune of course) and trained, is only (I'm not at home at the moment), about 100 or so years old. The year was like 132 or something when he appeared, and in legends it said he was born about 100 years back. Again, I forget the exact number but I'll look when I get home.
Assuming that in world gen those body size tags apply there too, is it safe to assume that all dragons I at least have ever encountered were only, as powerful as they were/are (thanks firebreath!), about 6000 in physical size? That's about half the size of a 1 year old dog, according to their raws also.
If that's the case, then all I can say is Holy Armok's Beard! I would also suggest that, if modding dragon children in, perhaps editing their size tags would be in order. Something like, at 5 years, they're size 10000, etc. Or do those body size tags work differently than I think? Does the creature grow in equal increments every singe year such that it reaches whatever size is defined by the next body size tag or something like that?