There are several categories for Under Counter refrigerators.
If you are going to be placing the Undercounter refrigerator between cabinets with the countertop directly above it you need A built-in capable refrigerator. These refrigerators are made with a special air circulating fan to remove heat from the back of the unit and expel it through the front kickpanel. Without this feature the refrigerator will overheat and fail to cool properly.
There are 3 sizes for built in units. 15 inches, 20 inches and 24 inches width.The typical height of an undercounter refrigerator is 33" - 34". There are also 32" height models made for ADA applications where lower countertops are required.
Click on the following links for the best search results: Need Help? Call 1-800-771-7702 Counter High Freestanding Refrigerators
Counter High Built-In Capable Refrigerators
ADA or Assisted Living Height Undercounter Refrigerators
Glass Door Undercounter Refrigerators
Undercounter Refrigerators with Drawers or for Special Uses Outdoor Undercounter Refrigerators