The 19 qt will keep a pound of minnows alive. That's as small as I would go. If you have plenty of space in the boat, the 30 qt is better.
Another option is buy the 25 qt Igloo marine and drill a small hole near the top for the aerator hose and get the Frabill aerator that runs on 2 D cells or 12 volts and you're good to go. I've never seen any of the Engel aerators last more than 6 months. I've used the Frabill aerator for 3 yrs with no problems. The Igloo marine cooler is insulated just as well if not better than the Engel.