My idea is that a fan is activated whenever relative humidity exceeds 65% or low than 55%... Is there any better idea?
Just moving air around won't change the humidity. You need to add or remove moisture or heat to do that.
I thought to put a resistor but i would like to know how to control it.
A small resistor won't do much in a 1/2 cubic meter space. If you DID use one, though, you can't make the Arduino provide the needed current. You need to provide another source of power that can supply the needed current, and have the Arduino turn a transistor on or off to control the flow of current.