Public lists
Germany is first, because that is the country of origin. The other pages are European club lists that name Mittelschnauzer kennels, then Russia. Each kennel line is a name, a place, and a link to the page that lists it. Nothing here is a ranking.

Standard schnauzers 
Pepper and salt 
In snow
Schnauzerringen in Sweden was checked. Its breeder-register page was not naming kennels, and its litter notices did not state the size, so Sweden has no page here.
Germany
22 kennels on Pinscher-Schnauzer-Klub 1895 Landesgruppe lists, under Schnauzer rather than Zwerg or Riese.
Austria
3 kennels on the Österreichischer Schnauzer Pinscher Klub Schnauzer list.
France
9 élevages on the Club Français du Schnauzer et du Pinscher Schnauzer moyen list.
Netherlands
4 kennels named by the Nederlandse Schnauzer Club for the middenslag.
United Kingdom
Club lists are public. They name members, not kennels, so no kennel names are repeated here.
Russia
3 kennels whose own sites say they breed Mittelschnauzers and give a city.