Sizes

Cases can come in many different sizes, or form factors. The size and shape of a computer case is usually determined by the form factor of motherboard that it is designed to accommodate, since this is the largest and most central component of most computers. Consequently, personal computer form factors typically specify only the internal dimensions and layout of the case. Form factors for rack-mounted and blade servers may include precise external dimensions as well, since these cases must themselves fit in specific enclosures.
For example, a case designed for an ATX motherboard and power supply may take on several external forms, such as a vertical tower (designed to sit on the floor) or a flat desktop or pizza box (designed to sit on the desk under the computer's monitor). Full-size tower cases are typically larger in volume than desktop cases, with more room for drive bays and expansion slots. Desktop cases—and mini-tower cases designed for the reduced microATX form factor—are popular in business environments where space is at a premium.
Currently, the most popular form factor for desktop computers is ATX, although microATX and small form factors have become very popular for a variety of uses. Companies like Shuttle Inc. and AOpen have popularized small cases, for which FlexATX is the most common motherboard size. Apple Computer has also produced the Mac Mini computer, which is similar in size to a standard CD-ROM drive.
There are mini-tower, midi-tower, big-tower/full-tower. Full tower cases are typically 30 inches or more in height and meant to stand on the floor. They have anywhere from 6-10 externally accessible drive bays with more internal-only. The ratio of external to internal bays is shifting however, as computing technology moves from floppy disks and CD-ROM to large capacity hard drives and network-based solutions. Midtower cases are smaller, about 24" high with 2-4 external bays. A minitower case will typically have only 1 or 2 external bays and stand from 12" to 18" tall.