A question from a fellow grower:
How can I create more humidity in a outdoor building that I built a grow room in… I covered the walls with clear plastic and placed a 5 gallon bucket in there with the plants will that help
Co2 or a humidifier ?
An open reservoir will definitely add some measure of extra humidity. How much can only be determined by experiment with how much you need to use. Warning: Too much dampness in the air can cause mold and mildew; So you need to be aware of what is going on daily.
Or; Buy a humidifier. 
our fellow grower adds:
Is a one gallon bucket t small to grow the plants in…if so what size should I use
I add water to a shallow tub with clay pellets in it to create as much surface area as possible.
No a one gallon is good long as you don’t let plant grow to long and become root bound .
Please let us know how much of a variance this allows you. Thanks