In reply to David.

Thanks David. All the A sizes are technically viable if you can get them to fit into the respective boxes, but from experience I don't generally recommend them for a variety of reasons including that some have the tendency to want to mix them in with US standard cards which can tend to cause some to go "missing" because the differences in sizes make them much more difficult to flip through quickly. Almost all the older literature recommends to use "cards of a similar (same) size" for this very reason. Because of small size differentials, they can slop around in one's box as well.

I tend to use my cards pretty heavily, so I stay away from thinner standard paper as it wears ragged over time. I'd certainly appreciate some thicker Tomoe River cards if they existed. Clairefontaine (Exacompta) makes some really nice fountain pen friendly index cards , but they tend toward the more expensive, and are—yet again—another size which doesn't match any of the other US or European standards.

One can obviously use any size and type they please, but knowing some of the pros and cons of those possible choices upfront tends to be most helpful, especially if one is going to have a lifelong practice.

Good luck with your practice.