I went to 4.5m - the rings did not slide easily any more as we paused for a year. If you work on it right away, you can go much further

I went with an inner diameter of 1m, this is fine for a small shovel, a pump, a small jackhammer.

Concrete rings are used all the time in road construction work. I don't see a risk of collapse with those when stacking carefully. Even if one ring were to get a crack, where should it go? I felt safe in this regard