I spent the day at Scioto Trails State Park here in Ohio. It has two 15 acre "lakes," one of which I spent a lot of time inspecting and a little time fishing (rain/snow and too damn cold).
The earthen dam is about 40 feet high and about 200 yards wide. The concrete spillway is probably 75 feet wide, leading into a 10' square drain tunnel under the dam. Interestingly, there is no secondary spillway. I guess ODNR should be pretty good at calculating max runoff and how much dam height they need above the spillway to handle a big flood.
This is the first "big" dam I have spent any time looking at since I got into pond management. I now have any opinion on whether a body of water this size is a pond or a lake: If you spend the money it takes to move that much dirt, you can call it anything you want.