When I do this kind of planning I gravitate to the 20' width also. One of the reasons I do is birds. I figure that 20' could be spanned by metal hoops holding netting, keeping out the birds, just leaving snakes, bears, and gators to worry about.
BTW, there are no producing farms I've seen that have 20' wide ponds. Well, ponds for tropicals I have, but not koi.
Hi.
Mickey the windowman