I have 3.8 cubic meter room. I believe the 6 kW is enough if I put 9 kw do I get any benefit I’m in more power

You should shoot slightly over but if you follow our online calculator, this will give you guidance

https://Posh.co.uk/blog/steam-generator-size-guide-calculator/

If you go too far over, you will simply engulf yourself and the room too much