Hygroscopic material absorption level
I tried to do something similar when I had a humid car.

In chemistry, we often use $\ce{CaSO4}$ or anhydrite to serve as a desiccant. You could probably get cost-effective plaster for similar purposes. You could also get gypsum and heat it to generate plaster of Paris yourself.

As you're probably aware, people also use silica gel in products (and food) to minimize water content. 

Either material should be much better as a desiccant than regular salt.

One key criteria is to increase the surface area - make sure the material is a fine powder and enclose it in something porous. I've seen people use various cloth materials, paper towels, etc.

As to how much you need, you can work out how much it will eventually absorb. If I take plaster of Paris to gypsum, each mole of $\ce{CaSO4}$ will take ~1.5 moles of water. So for every 136.14 g of anhydrite or ~145.15 g of plaster, you can absorb ~27 g of water. (I use ~ here, since I'm not convinced that regular plaster of Paris will be completely $\ce{CaSO4*0.5H2O}$.. some will likely already have extra water in it.)

The trick is that humidity will continually increase in the room if there's exchange of outside air.

Personally... I'd go with an electric dehumidifier.