Microwave size is determined by the cubic feet of the oven's interior cavity. A small microwave will measure less than 1 cubic foot; larger microwaves can measure 2 cubic feet or more. Knowing your microwave's size will help you determine whether you can cook large items, such as a whole chicken or turkey.
Open the door of your microwave and measure the length, width and height in inches of the interior cavity.
Multiply the length by the width by the height using a calculator to get the total in cubic inches.
Divide the cubic inches by 1,728, which is the volume of a cubic foot. The resulting number will be your microwave's measurement in cubic feet. You can also use an online conversion calculator to simplify matters.