Microcomputers are small computers. They range from small controllers that work directly with 4-bit words. It can address a few thousand bytes of memory to large units that work directly with 64-bit words and also can address millions or billions of bytes of memory.
The scaled-down versions of mainframe computers are often called minicomputers. The main unit of a minicomputer usually fits in a single rack or box. A minicomputer runs more slowly, work directly with smaller data words, and does not have as much memory as a mainframe.
The largest and most powerful computers are called mainframes computers. It may occupy an entire room. They are designed to work at very high speed with large data words and have massive amounts of memory.