I like to make my holes about an 1/8" or maybe a little bigger. I just use a soldering iron to poke them. I make all my holes on the side of the tub. That allows for a good amount of ventilation, without a huge loss in heat or humidity.

Poke as many holes as you need to maintain the correct humidity with everything you normally have in the tub + heat. You shouldn't need to mist or do anything extra to help with humidity in tubs since they hold it so well.