The largest true arvicola seems to be the yellow-cheeked mouse of Hudson's Bay, and the biggest representative of the family on either continent is the muskrat.
"Camping & Tramping with Roosevelt"
John Burroughs
Myodes obensis, M. torquatus, and arvicola obscurus.
"The Voyage of the Vega round Asia and Europe, Volume I and Volume II"
A.E. Nordenskieold
Differing from both these species, arvicola obscurus does not appear to show itself above the snow during winter.
"The Voyage of the Vega round Asia and Europe, Volume I and Volume II"
A.E. Nordenskieold