Opinion here - go for the biggest radius circle or oval that will fit your space, then double- track it, then pull a branch line off at any radius you want and create your switching yard, maybe on another level.
That's what I did - 14x28 - and the giant 17/64 scale 4-10-2s can actually run double-headed with 80 foot cars. I still have plans for the yard, but no patience to sit and switch cars. Best I can do is hold a Margarita and watch.
