The rules in MotW are the same as those given in the DMG. You can have up to twice your caster level in HD of companions. However, if you adventure, you can only expect to keep half that amount. In any case, no single companion can have more HD than the player. In other words, a second level druid could have two 2HD animals, so long as he doesnt go adventuring, but he could not have a 4HD animal even if all he does is stay home and take care of it.
The thing that stumps me is that a wolf (2 HD) is considered to be a good selection for a first level druid--it is even suggested in MotW--even though it would be a violation of the rules. Can anyone figure that out?