The big boss at Walmart says the company's supercenter stores are just too big.  CEO Doug McMillon says the world's biggest retailer found out "how big was too big" when the company learned that some customers don't want to walk around a 200-thousand square food store to do their shopping.  That's how big Walmart's biggest supercenters are, the size of four football fields.  McMillon told PBS interviewer Charlie Rose that Walmart's been building smaller supercenters over the past few years.  McMillon says Walmart's future stores will keep getting smaller as the company focuses more on its e-commerce business and adds more Neighborhood Markets, which are about one-fifth the size of supercenters.