You wouldn't wear work gloves that are too big and clumsy, or boots that are too tight. You also shouldn't wear a respirator that doesn't fit.
Respirators designed for use on the farm come in many shapes and sizes.
Select one that fits your face well. Remember that anything that can slip between the face piece and your face will likely make its way to your lungs. Use extra caution if you have glasses, a beard or mustache, and long sideburns. If you're concerned about how your respirator fits, take it to a professional or hospital clinic to have it fit-tested. Your lungs need all the help you can give them, especially on the farm.