From what the OP is describing, a 3-4kW heatpump will be more than enough. the lower the output the higher the efficiency. check the COP. my 3.4kW heatpump has a COP of 5, and a lowest power consumption at 200W (presumably delivering 1kW of heat). but disclaimer - YMMW! [i am basing this on your room being closed not open plan]
my 6kW heatpump has a COP of 4.4, can't remember what the lowest power consumption is. the 5-6kW range then to be cheap as they have many sales. presumably 3-4kW also cheap if there are sales. if you are afraid of too overspec, then check the lowest power consumption the high kW heater can do.
Thanks, I'll check these specs.