I am ready to build a root cellar. We have a very large hole excavated. I have several options, which include burying 2 pre-fabricated concrete tanks or build my own. The pre-fab would be quick and give us about 12 x 16 space with 2 separate zones. My question is how much can you store in approx. 180 sq ft? I don't want the space to be too big but I do want it big enough to grow into it, and I definitely don't want to invest in something we will grow out of in a few years. We plan on selling winter vegetables through a winter CSA and select from the storage crops for biennial seed production. Any thoughts would be helpful.