I agree with Uberstationmeister
Big ones look to be O72
Other sizes might be O42 but probably O54
To be sure:
Make a circle (or half circle) and count pieces 16 pieces make a circle in O72 for example. Like you suggested.
Also note the number of ties per piece (But here is variance between manufacturers example Menards has less ties per radius in O72 if I remember.) Keeping the same manufacturer on all track will help if possible but not entirely necessary.
Then label a reference piece in each size. I wrote out labels and stapled the labels through the rails. It helped a lot in the layout phase.
031 is a pretty small piece so easy to tell. IMO O42 and O54 are the hardest to distinguish though the number of ties may be different. Don’t ask me how I know this.
Hope this helps
Steven T
