The correct sizing of a heat pump is very important from two points of view: the cost and the coverage of the heat losses of the building. If you oversize your heat pump, you may end up with higher heating or cooling costs than you would otherwise have. An undersized heat pump will not provide enough energy to cover the heat losses of the building, or will use an additional heating source (electric heater) too often, which will ultimately lead to higher costs again. 

Heat pump design and installation experts have the right tools to calculate the correct output of a heat pump, so it's best to leave it to them to determine the correct output.