ie: no individual should have any business having more than 10 million in cash and 250 million in assets, or whatever.

Kim Stanley Robinson goes into a great deal of detail about this idea and other similar economic ideas in his Mars trilogy. For example for large projects where you would need a great deal of capital, companies(which are limited in size by law) would be allowed to form temporary cooperatives for the duration of the project(subject to approval and oversight, of course). It’s definitely worth reading.