Switches should be sized for the largest engines and rolling stock you will ever possibly have. Everything made to date will navigate O-72 so that would be a safe selection for a minimum size switch, if you have the room. Your curves will have to be O-72 minimum as well. Bigger is fine and also better. The largest size you have room for would be the best way to go.
I have only modern diesels, no steam, and limit everything to O-54. Most all diesels will navigate O-54, so I should be good. I believe there may now be some rolling stock that requires O-72, so I will have to watch out for those and be careful not to purchase any. If I ever get the urge for a large, scale size articulated steamer, I will have a big engine and even bigger problems with my layout.