It's the Longboard Forum, go the Long one, 6 foot!

Whether it is 5'6 or 6'0, you'll still be carrying it! So weight doesn't matter for you. He'll let one end drag in the sand, so weight won't matter for him either!
6 foot will give better glide, more instant fun. More likely to stay with the reforms longer.... more stability (probably).
But either would be fine.