While external dimensions vary, experts group microwave ovens in terms of their intended placement:
Countertop microwaves:
0.5 to 1.9 cubic feet of oven capacity
Over-the-range microwaves:
1.4 to 2.2 cubic feet of oven capacity
Commercial microwaves:
1 to 1.5 cubic feet of oven capacity
Built-in microwaves:
1 to 2.2 cubic feet of oven capacity
Each microwave size also varies in wattage.