Even 8kw is probably too much. And this is from someone with kfw70 and 250m2 living in the northern Black Forest who just came back from shoveling snow.
Find out what the standard outside temperature is for your region.
For example, for us it is -14 degrees.
Then look for an online heating load calculator, roughly enter the values and increase by about 20%. I think you will end up somewhere around 5000-6000kw maximum.
But if you are unsure, invest the 300 euros in a professional heating load calculation.