I was thinking of something around 2500-3000 gallon. Plastic ones of that capacity are pretty big - something like 4 x 8 x 12 feet and a thousand pounds or so. But if you're thinking of one you could haul in your pickup, then it's maybe 25% that size (at least that's as big as *I* would try to haul in a pickup). At that size, the plastic one probably wins, given the effort and cost of transporting stuff to your site, ease of plumbing, etc.
The Ames Research Laboratories "Blue Max" videos are pretty interesting in any case. Their test tank looks to be sitting on a concrete pad and seems to be about 40" x 48" x 30" interior dimensions, which would be around 250 gallons. Their other video shows a test concrete block building that's about 10 x 12 x 10 feet with a flat plywood roof. In both cases, they don't use any mortar, nor do they use any nails in the roof. Only the Blue Max coating is holding everything together (along with making it waterproof), which is the part that really interests me. I may actually have a use for this stuff.