Your feed should be kept in a cool, dry place that is as rodent proof as possible, and out of the sun. Direct sunlight will destroy the nutrients and vitamins.
Be cautious when using metal containers as in some climates, the metal will sweat and ruin the feed. If using a covered plastic storage container, a 10 gallon container holds 50 pounds well, but shouldn't be used for longer than 3-4 weeks since feed will still loose nutritional value as it ages and will spoil.
You can read other helpful feed storage ideas here.