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.
i know this is a Rather old Post, but couldnt find any other posts about this..
you Mention Stacking RC tanks..
i got atm, 4 9x9x8 tanks next to eachother, and would love to stack those into on (have my base at lvl 72, so could have 4 of these below.
could you "explain" how i do this, as i tried to place the valves on each block of the "test" tank i made, but it counts them as 2 seperate tanks, and no fluids is transfered between them