It kind of depends on how you want to use it. If you prefer to straddle the beam, and pull towards you, shorter is nice. But if you are like me, I like to stand behind the beam and lean into it. Therefore I like a long beam, so I don't have to lean over very far to reach the lower end. Mine extends from the floor to just above my belt buckle.
For years I used a sycamore log ripped in half, with a piece of carpet tacked over the working end. It was about 12' long and 10" in diameter. Just be sure to find something that you are comfortable with. There is not much worse than spending a few long nights hunched over a short board trying to flesh a bunch of beaver. Your back will be screaming at you for weeks after your trapping vacation.
But don't worry, by next year you will forget and be doing it all over again 
