Don't use an intake fan, they are a waste of energy. When an exhaust fan moves air out of a room, the pressure drops slightly and an equal volume of air is pushed in from the higher pressure outside, if you have a passive intake (hole) roughly the same size as the exhaust.
Cfm stands for Cubic Feet per Minute, and is a measurement of how much air a fan will move. An alternative is cubic metres per hour, m3/hr. Some fans will have it printed on the label with the wattage, voltage etc, others don't and you'd have to ask the place you bought it or check the specs online.
You need to know the airflow of the fan so you can work out how big a fan is required for your room, it's a bit hit and miss to just guess.
What is the humidity outside of the room? If that is ok, then you could well be overventilating. If the humidity in your area is low anyway, then the simplest way to raise it is to have a bucket/bowl of water in the room with a towel hanging over the side and a fan blowing across it. If there is not space inside the tent then sit it near the intake hole.
Also check your nighttime temps, you should be keeping them above 18c to avoid stress to the plants.