It might be easier to buy a pattern.
Watch for the sales at JoAnn's or Hobby Lobby.
I'm thinking a rectangle about 17 x 25 might work.
Logic for that guess: 10 + 3.5 + 3.5 for the width. (10 inches plus 3.5 on each end for boxing)
6 + 6 + 6 + 6 = 1 for the circumference plus the allowance for the zipper.