Hmm. For the tail I'd suggest using parts of a red and yellow feather boa.
CanineHybrid on here (not to mention YouTube and DA) made a Charizard pokemorph and used the feather boa idea... I think she reinforced it with some wire, too. I'm not sure.
Length depends. If you're willing for it to drag on the floor behind you, you need to remember that it'll collect lots of stuff from the ground. If it's really short, it'll stick out.
So it's really whatever you feel comfortable with.