I run an ebb and grow, with 24 buckets. I keep my res at 40 gallons, and it's plenty. I do use the smaller buckets though, never had a need for the big ones. It's always best to have more water than needed to fill to help with keeping ph and ppm's more stabil, just a bitch to deal with that much water.
If you're using drums, I would suggest using a 2" pipe system that has a 90 and a bigger sewer type top and screw in cap, so you can connect a hose the same size as the one leading to the controller bucket, and run that hose into the drain that goes outside for changing out the water. It saves a lot of work. I use a bigger top with screw in piece so I can place a big funnel in it to pour out buckets of water from cleaning, and whatever else. This will save you a shitload of work.
I have a lot of other good tips for the construction, so feel free to PM me for any info you want.