Question: Under the stairs that take to my bedroom there is a tiny room, as big as a cupboard.
I can barely stand in it. 
I smell mould in it, I tried keeping its small door opened and I think situation vastly improved. 
With door closed I get 80% / 90% humidity and with door open around 50% / 60%.  

For this reason I would like to improve its ventilation and I thought about cutting the door to put a vent with or without a fan or just shorten the door. 

Am I going in the right direction?

Is there a safe and easy way to calculate the fan / vent I need? 

Here is a picture of the entrance:
[![enter image description here](https://i.stack.imgur.com/50eRu.jpg)](https://i.stack.imgur.com/50eRu.jpg)

And a picture of what could be the source of humidity, a non complete part of the floor where I see dirt (but I am not sure) on its top you see the beginning of the stairs:
[![enter image description here](https://i.stack.imgur.com/h7FM3.jpg)](https://i.stack.imgur.com/h7FM3.jpg)

Thanks for your help! 

Edit after comments:
In the room one wall is communicating with a tiny toilet (closed, no windows) another one is shared with my neighbor (I guess, I don't know if there is any space between us), one is the one with the small door you see and then there is the one where the stairs are.

I also attach a couple of pictures that are often worth much more than words

Answer: 
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 & 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.
If there is an outside wall inside this space, I would insulate that wall with styrofoam sheets glued to that wall. Then I would go to Lowes, Home Depot , or a large hardware store and ask to see a "through the wall" fan. Install the fan where convenient and install a small heating system grill on an opposite wall. The fan will ventilate the room and add some heat from the heated area of the house.