So I grabbed a 'few' from my collection for a ring size measure... just to be sure, then a few packs of 100 sheets of plain white Filofax paper and then try each size of rings with different numbers of sheets.
You will find the results in the FAQ Page under 'R' for 'Rings' with a 'Capacity' column added:
This is just paper only, no dividers or anything else, so in reality you might have to deduct 10-15 sheets depending on what else you have.
Ring Size Organiser Size Capacity (Sheets) 30 mm A5, Personal Zip 340 25 mm A5 Clasp 280 23 mm Personal Clasp 250 19 mm 200 15 mm Compact 150 13 mm Slimline, Mini 120 11 mm Slimline, Mini 105
If the smaller ring size look out of proportion to the larger ones. It is because with the smaller rings the page offset is the same so they are harder to turn on the smaller size rings. As you get above 15mm the offset isn't so critical.
And the 25 and 30 mm ring size might be able to take slightly more than I have measured, but I've erred on the safe side of things.
Of course if use Filofax Cotton Cream Paper then it is thicker so you will have to reduce the number of sheets accordingly.