Since most of my toys are made to fit our bodies, the majority are constructed with oval cross-sections making the diameter vary. Anyhow, your hole really doesn’t care how ‘wide’ something is….it sees things in terms of how far around it’s going to have to stretch ‘around’ an object.
And for my irregularly shaped creations, circumference gives a much more accurate reading of how big you’re going to have to stretch to get around it. Also, being hand-made objects, even those that may appear spherical are still not perfectly round.
To convert circumference to diameter simply divide by 3.1416 (Pi). Just be aware this will be average diameter of the toy.