Good question, but I don't think there is currently a perfect answer to this. Google Maps doesn't know if one supermarket is more "actual" than another. What would be the criteria? Even if you wanted to draw up a list of "actual supermarkets" it would be very subjective. That's still what we do with options 2 and 3.
Search for a supermarket that is a superstore
supermarket superstore and you will be sure to get big supermarkets. However you won't get the medium size ones.
Option 2 (gives good results)
Search for the known supermarket chains, remove the ones which may be categorized as "convenience stores" or have "local" or "express" in their names.
supermarket AND (superstore
OR aldi OR asda OR budgens OR cooltrader OR "the co-operative food" OR farmfoods
OR "fulton's foods" OR "heron foods" OR iceland OR lidl OR "mark's & spencer"
OR morrisons OR ocado OR "sainsbury's" OR tesco OR waitrose)
-local -express -convenience
Tried it for my area and I can't see any small shops.
If you are in a hurry, on mobile or if you don't like the idea of using brand names (because that's a list to update with time), we could do shorter. We would get more results, but little shops would come back as our problem is that a place named "Supermarket" in the category "Supermarket" will appear no matter its size.
supermarket superstore -local -express -convenience