Basically, by weight.
Is it fresh or frozen? I assume its meat you just killed.
This summer - on one of the hotter days of the year - I had a friend send me 30 lbs of CAB sirloin, ribeye, triptip and porterhouse. Put it in a styrafoam box and packed it tight. Sent it 2nd day UPS and I missed the delivery. Ended up picking it up at the UPS depot 50 hours after it was shipped and most of the meat was still frozen solid, with the stuff on the outside cold but thawing.
Not a problem.
I would suspect if you were shipping freshly killed game back it would be wise to pack with dry ice (though check with the shipper - at least airlines out of SD these days won't let you ship pheasant with dry ice as the CO2 can kill the dogs in the back of the plane). Last year I packed a cool with 15 pheasants, with a layer of ice on the bottom and layers of ice between each layer of pheasant. Filled it tightly, then covered the top with a towel and duct taped it up. Shipped it via cargo on the plane it it came through fine.