That was sad news when Rusty had to dismantle the Great Plywood Glacier. We enjoyed seeing the pictures taken on that layout.
20'x24' is a generous size room for an around the walls layout. Mine is 17'x21', I chose 30" as the minimum radius but I am not using body mounted couplers or running large River Raisin engines. If I had a 20'x24' space I would have used a 36" minimum radius exactly as Rusty advises. All my mainline turnouts are #6 or #8. Only freight sidings and the freight yard use #5 turnouts.