Don't count out small tank cars--they have come in just about every size. An 8,000 gallon tankcar is very small in 1/48 scale.
Don't discount Lionel's offerings. The pre war and post war tin cars are scale size if lacking in ultimate detail. They would make a good starting point.
Right. For comparison, here are a few shots of an Atlas O 8,000 gal. scale tank car, a Lionel traditional tank car, and a Lionel 8,000 gal. scale tank car.
The Atlas O car and the Lionel car are virtually identical in size. (And actually, the more recent Lionel cars have are slightly more detailed, with additional rivet detail.)
The Lionel scale 8,000 gal. car, while shorter, is still an 8,000 gal car, because in this design the tank is wider, as shown in the photos comparing it with the Atlas car.
