Hydroton/GH Grow Pot question

Planning on switching from 6" rockwool cubes to meshed grow pots with hydroton in them. Would 6" round pots be the best size? (plants will grow to between one and 2.5 feet tall... 2.5 is kind of an absolute upper max). With the pots, the roots should just grow out as they please right? So no risk of root lock?

How much hydroton would be needed to fill 70 such 6" round pots?

These are the pots I'm thinking of: http://www.horticulturesource.com/c-a-p-custom-automated-products-6-net-pot-24-pack--p9182/?osCsid=b432eec11ca99352df379d08bb85d93e
 
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