What is the correct size of garbage disposal that I should put under my sink?

If you are replacing an existing unit that has worked well for a number of years, the old size should work fine. Otherwise, 1/3 horsepower will handle limited use once a day. A standard 1/2 horsepower will normally suffice for general use. Heavy-duty 1/2 horsepower units and larger are designed for a large volume of garbage.