I don't see any problem with very large cooperatives. In fact the optimal structure would probably be to leave everything that isn't a public good, natural monopoly, &c. to cooperatives and let the market decide what firm sizes are most efficient. (Use antitrust as necessary.) Let everyone have equal rights to the share of capital and the right to form cooperatives that they think will use the capital most efficiently. Let the central bank, possibly automated by public referenda, set the savings rate, and from there let people make loans to each other to control at what points in their lives they'd like to consume.