The maximum default tank size is indeed 7x7x8, storing 6,272 buckets (which each bucket being 1000mB, or "millibuckets", of liquid which is where enoch is getting his millions from)
You can increase the maximum floor dimensions from 7x7 to 9x9 in the config, but the maximum height of 8 blocks cannot be changed. So the largest possible singular iron tank is 9x9x8, storing 10,368 buckets.
However, you can stack iron tanks vertically if you place valves in the right slots, and they will combine their inventories. So in practical application, the largest possible Railcraft iron tank is 9x9x248 blocks large, storing a staggering 321,408 buckets of liquid. The world height limit of 256 and the bedrock layer starting at 5 prevents you from stacking any more tanks on top.