there are different types...but the pack usually comes with two large two medium or something similar.
you use the hose on your home vac to suck the air out. i'm sure the ship has a vac you can borrow from the steward if needed...i've heard of others doing this.
these are the ones that you can roll the air out of. i usually roll on top of the bag ::insert visual image and lots of laughing here:: wal*mart sells the kind you vacuum too.