There aren't any specific square feet guides. I think the RSPCA have changed their recommendations when I was making no pies I found this quote, but the link doesn't work now. "Their accommodation should have a sheltered sleeping area of at least 6ft x 2ft x 2ft with an attached 6ft x 3ft run to allow them to sit up straight and look around."
The above measurements give an area of 30 sq ft (I think). I don't personally think it matters if the ratio between hutch and run doesn't match, so long as the square footage is up to par.
So I guess as those guides are for 2 buns I would work on providing 15 sq ft per medium sized bun.
Having had a good look I found a pdf file from the Rspca stating: 180 cm(5.9 ft) X 90 (2.9) X 75 (2.4) high for two medium sized buns and none given for run size.
I saw someone quote something from the RSPCA the other day so maybe I'm not looking in the right place?