Thanks for contacting us! Jim's out for the week, but I can definitely help you out. A five head should do well for you in both cooling, and humidifying.
A 55 gallon drum would be perfect. The more water you have, the less the water is heated by the mister. Which is a good thing.
I want to say the temp in the people that have reported back have had around a 10-12 degree drop. I say with with caution though. I've never done it personally. The more water you add, the cooler it'll be for sure. If you want to add a bigger unit, you could add a humidistat to control the whole thing. Another benefit to the bigger unit is more stable humidity levels. With an air exchange, it'll snap back very quick.
If you have any other questions, I'm happy to help.