Hi ox2015, please read these lines http://wiki.openstreetmap.org/wiki/Tag:shop%3Dmall and to specify the levels or floors read these as well. And yes it’s a lot.
Ps take a look at some of the malls in your region, country or elsewhere. (search for a large parking space with a building in the centre or nearby. For instance this one http://www.openstreetmap.org/#map=17/39.14759/-84.44688
You cannot draw the outline of a shop without knowing the outer lines. The backside of a shop is or could be unclear. And you don’t get the knowledge or an invitation to look around.
So try to add a tag as close to the shop area as possible, count steps or some alike from a welknown point.