The mini ones are talking <15ksf, really limited selection of foor and merchandise, aimed at quick shopping trips but I would imagine a better selection and management than Dollar General. And this is coming from someone who regularly shops at DG (best place around for generic snacks and nuts etc). The original article is pretty interesting in how they will have it tied into their distribution system, you can order anything in their whole system and have it delivered to the small store.
40ksf is more like their Neighborhood Market, full line of groceries with limited merchandise, same as any supermarket. 90ksf referenced above is pretty much the size of their original stores before they added grocery to make it a supercenter.