Comparing them to a known size is the best way but here's a guide I've used to sort out unknown sizes. Don't remember where I got it, but it seems close.
size - height - diameter
2/0 - 0.235 - 0.157
1/0 - 0.265 - 0.180
1 - 0.292 - 0.199
2 - 0.325 - 0.221
3 - 0.370 - 0.239
4 - 0.420 - 0.276
5 - 0.452 - 0.315
6 - 0.475 - 0.354
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