It may be big by it sells a product that other companies want. When there's a market for your goods then you have someone in the background willing or waiting to take your place. It may be large but lets just count the US market. It may have a short term effect on the US but the US will recover not like the bank failure of 2008.
I gotta think that, whatever it would take (U S economy-wise), to precipitate a Walmart collapse, is the real question. Now, if the upper echelon at Walmart decided, suddenly and without prior warning, to shut the doors, then I think there would be a world-wide effect. This effect would only last as long as it would take the other Walmartwannabes to gear up and fill in the holes. This "gearing up" could take some time...how much time?...be ###### if I could even throw a dart at it.
(It would be a great time to invest in Target, KMart, and CostCo!!!!)