You waste energy if you have an air conditioner system that is more powerful than you need, but you also waste energy if your unit is not powerful enough. An air conditioner that is not powerful enough will still use a lot of energy, but the energy won’t be enough to cool down the room very much. That would be wasteful.
The power of an air conditioner system unit is measured in tons. If you say you have a one-ton unit, that means it can remove one ton of air from the room, cool it, and return it to the room, in one hour. To make things more convenient, many countries use British Thermal Units (or BTUs) to measure the power of air conditioners in a standardized way.
The size air conditioner system that you need will depend on the size of the room or rooms you wish to cool down. One of the air conditioner installers will be able to evaluate your space and let you know which size would work best.