How can I ventilate efficiently a tiny room under my stairs?
I believe I see concrete, if a ground floor with concrete this may be your issue to know for sure tape a piece of plastic to the floor for 24 hours . When you pull the plastic up if it is wet you have found the source of the moisture. 

I have had good results using 2 part epoxy paint on concrete sealing damp concrete. I usually wait until summer when the moisture is lower. I prep the floor with a muriatic acid &amp; water etch after that is done and the concrete has dried (a fan blowing over it helps to dry faster) I coat the area with the 2 part concrete floor epoxy this has eliminated several homes problems with basements and daylight basements that had the same issue.