what size heat pump do I need
Heat pumps or air conditioners are the same thing just called different names around the world.
They both heat and cool and circulate air.
The easiest way to size a room is to us this calculation.
width x length x height x 0.05 + kW or (heat pump size)
eg. 5m x 6m x 2.4m x 0.05 = 3.6kw this would mean you need a heat pump or air conditioner that is rated 3.6kw or above in both heat mode and cooling mode.
Don't be fooled into getting a system that can only meet the heating as if the cooling come's under this you have no warranty.