A general rule of thumb is that one cubic foot will hold about 30 pounds of meat, depending on how you store the meat in the freezer. That means 300 pounds of meat should fit in a 10 cubic foot freezer.
Getting a larger freezer allows for more food to be stored, but having dead space in a freezer will increase your electricity costs.
One other point: for long term storage you are better getting a manual defrost freezer. They're harder to find, but will keep your food fresh longer because the auto defrost models go through a warm-up cycle to remove frost. I've kept meat in manual defrost freezer for more than two years with virtually no degradation.
Put one of the wireless thermometer sensors in the freezer, and put the remote readout unit where you can easily see it. Keep the meat at or below zero for best storage.