Kitchen configuration and household size are the two most important factors to consider when choosing the cubic footage of a microwave oven. Small apartment kitchens that only leave room for a microwave on a counter don't allow for as large a microwave as a larger kitchen in a single-family home.
Microwave ovens range in size from 0.5 to 2 cubic feet. If the purchaser is a single person who mostly uses it for single-serving entrées, a smaller microwave is suitable. However, people with large families or who plan to use the microwave as part of preparing for large dinner parties need a larger oven. A single person or couple might choose an oven up to 1.3 cubic feet, while families and people who host large parties may choose ovens closer to 2 cubic feet.
The placement of a microwave plays a role in its size as well. A countertop microwave is likely going to be smaller as it has to share space with other small appliances, such as toasters and blenders, and everything else that goes on a kitchen counter. Built-in microwaves, typically part of a cabinet installation, can be larger, and over-the-range units can be even larger because they also share space with the exhaust hood above the range.