There are lots of tips on whoopies.. The original Whoopie thread will contain your answers.
Generally speaking, a popular size is 6 foot finished length.
To construct two whoopies at 6 foot, you'll 26-27 feet of new cord.
You lose around a foot to foot and half of length with the buries/splices.
Always use a strap on the trees.
Ambulo tua ambulo.