When I was a kid (46 now) we spent every summer at Douglas Lake. It is in Pellston Michigan about 40 miles south of the Makinaw Bridge. The University of Michigan owns half the lake for their biology studies, so it is never crowded. My guess is 5-8 miles across and fairly round. You can walk way out before getting over your head and the lake is like glass for skiing. There are many cabin rentals on the lake with five to twenty cabins.