We drive & have used 165 qt Igloos or similar for years. A large cow with lots of ice tends to just about fill it. Drain often & refill with ice - the wet aging has done well for us. A big bull will overfill the 165 if not boned out certainly but 1/2 a 165 works well - room for a decent sized buck in the other 1/2. The driving around & draining seems to do a good job of aging& cleaning the blood out of the meat. We almost always have to quarter & cut them up in the field & haul them 1-4 miles to the truck.
If you get one early & have it processed there slightly smaller volume & use 10lb dry ice to keep cold about 20 hours. Your cooler efficiency will play a role but even cheap coolers have worked & meat stayed frozen.
