William Bronson wrote: If you intend to bury the tanks, a few layers of used carpet might be all you need to protect the liner. I would be inclined to make the tank long and narrow, to make covering it cheaper.
Most building materials get cheaper(or free) the shorter they are.
Each tank you build is another set of fittings, so one large one could be better, on the other hand that would put all the all eggs in one basket.
William Bronson wrote: "Round is sound"
I wasn't sure thus would matter much if the tank was basically an in ground pool. Not much chance of the sides collapsing, at least not outwards.