From a scientific point of view, you should get 5 pounds of CO2. There is a tare weight stamped on each cylinder and the person filling it should keep filling until there is 5 lb inside.
The temperature of the CO2 in the cylinder will start off identical to the temperature of the surrounding environment. When CO2 transitions from liquid to gas (vaporizes), energy is required and the temperature will drop. When CO2 is converted from gas to liquid (using a compressor), energy is added and the temperature will rise. This process explains how an air-conditioner works.
The pressure inside a CO2 tank will depend on a few things. I initially thought it only depended on the temperature, but like so many other things, it's a bit more complex than that: