Where in the world are you? When I housed my Russians outdoors in southern California I used 20'x6' and 17'x8' pens and even those weren't big enough. I'd say go bigger than that! I used plywood walls 12" tall and a 4"perimeter lip that prevented escape.
Offer both sun and shade, have lots of plants and some underground shelter. Don't forget a shallow pond or large water dish, too.
I agree.
What location and how much room do you have?
Is it a secured, fenced yard? Etc.
Russians can both dig and climb. (and fight if not given enough room)