How do you define Small Cell, Pico Cell, Nano Cell?

Question

If I am correct then pico cell has large coverage area than femto cell. But I could not make out what to use when ?

Answer 1

A type of cell is known by its coverage & capacity. Small cell, which has smaller coverage than base station (Macro cell). There are three types of cells comes under small cell category known as micro cell, pico and femto cells.
Each of them fits into different types of requirements.
Few points about them as following:
Micro Cell has coverage of less than 2 kilometers.
Pico Cell has coverage of less than 200 meters.
Femto cell has coverage of less than 10 meters.

Reference:
http://en.wikipedia.org/wiki/Cellular_network