My recording room is about 9x5 meters.
how to place a drumkit for best room sound?
in the lenght or in the width ?

You'll have to try it and see. I would think that this would depend on many things, including the acoustic treatment in the room. If it helps, my main room is approximately the same size as yours (I have a hard time coverting feet to meters in my head - it's 18 by 28 feet) , with a ceiling height ranging from 4 to 6 meters, I've placed the drun along both the long wall and the short wall to good effect.

The ONLY way to go in these situations is suck it and see.
Move the drums until you find the sweet spot - hopefully there will be one.
I've done sessions where the studio engineer has directed me to set up my kit "in the usual place" and the producer has wanted me to move the kit around the room. We've often found a great sound - to our ears at least - in a spot where no one's thought to put drums before.
So you never know.
Try it with the widest part of the room left to right of the kit though first, as opposed to front and back.