I use
this one in a 4x4x7 tent. You can set the humidity to the level you want to keep and it will turn itself off when it gets to that humidity. I've noticed a minor discrepancy between what the humidifier says the RH is and what my thermometer says, but they're always within 5% of each other. You can (and probably should) also turn the light off on the front of the unit.
One thing you're going to want to do with a humidifier that I didn't think of when I started is have a cleaning schedule. CLEAN IT ONCE A WEEK! Put some distilled vinegar through it for 15 minutes once a week or else it's going to stink so bad you're going to think something is dying.
And never use tap water. Minerals in the tap water will not only dust your leaves, they'll also clog your filter/pre-filter so fast the air will stop moving in your tent within a couple of weeks.