I use 24". You will have to overlap 2 pieces of 18" or 24" to wrap a full-sized packer. However, with a 24" roll you only need a single piece to wrap ribs and butts.
You can use a single sheet of 30" butcher paper to wrap a brisket, but a 30" roll becomes a pain to store and transport for me.
What ever size roll you decide to get, I highly recommend getting one of these: