Many thanks for the responses

...I did go to the Carrefour site, but I could see nothing that tells me where the stores are located in Madrid. The virtual visit to a Hipermercado is fantastic!
Can anyone tell me what subway stop one of the big ones might be near? I'll be staying at the Europa at Puerto del Sol and the underground is
so easy to use. I will be in the area March 14-19, after a week near Avila.