When water is heated it expands, when it cools it shrinks, an expansion tank allows this to take place without adding or losing water. The amount of water determines the size of the tank.
The history of expansion tanks started with the need to raise the boiling point, to eliminate corrosion from oxygen and to eliminate the need to add water. The early hydronic system held a very large amount of water and required a large expansion tank. They also needed to be mounted above the boiler and plumbed to trap the air coming out of the boiler.
The insulating values of today's homes has greatly reduced the size of the boilers required. Very few homes need more than the smallest expansion tank.
The expansion tank is made out of steel and should not be screwed into a copper fitting, if you must install it on copper put a brass coupling between them.
The mistake most often made is installing the tank upside down. The pipe fitting must be on the top. The tank is made out of very thin steel and any air trapped in it will greatly shorten it's life.
Do not place the tank to close to the pump, it can cause the fill valve to pull in more water than the system needs and cause high boiler pressure.
When the tank is installed upside down it can be the source of noise in the system. When the system heats up, the bladder moves up, allowing the tank to hold air. When the system cools, the bladder moves down forcing the air into the system to circulate and cause noise. Also when water is heated it releases other gases besides air and these can be corrosive to the tank.