A couple questions:

You buy it once and all of your alts on one account get it? Does everyone get something in the mail like the Collectors Edition pets?

How tall is it when used as a ground mount? Elekks are so tall that my Draenei hits his head on doorways and stops instead of riding through and getting dismounted like all of my other alts (including Draenei on horse and cat).

Yep, it's all account bound characters, current & future, regardless of server.

It seems to me to be about as tall as a normal horse, maybe a little larger, but nowhere near the size of the Elekk or Mammoth.