As a general rule the rating of any switch and RCD should be above the maximum current the circuit is expected to carry, and in most cases that can be considered as set by the upstream OCPD (i.e. fuse or MCB).
Generally you find that for ratings in the 63A-100A for domestic equipment there is little cost implication so going with a comfortable margin (say just selecting the 100A as worst-case for most setups) is not too difficult.
If you are looking at sub-main (like a garage circuit) your starting point is always: "What is the load going to be like?"
If it is a typical garage it is a junk store
So you have the occasional 13A socket use and an amp or so lighting, maybe a few amps longer term if there is a freezer out there. But some folk have hobbies or workshop usage and that can push up the load, or massively do if it is an electrical stopping point to an external hot-tub or similar.
Once you know your load, you can think of the cable route, and from the length and environment select a cable that can carry the current with acceptable low voltage drop. You also have to think of earthing arrangements, as if the house is on a TN-C-S system
and you have extraneous conductive parts to bond your earth has to be 10mm copper equivalent minimum.
Finally when you have selected your cable, determined the end point Zs for fault clearing current, you can choose the OCPD to protect it against short circuits (and maybe against overload, though the downstream breakers might set that limit). Usually a fuse for upstream OCPD is going to give better selectivity than a MCB, but your use-case and cost/effort/etc might allow that in some cases.
Finally you need to consider Part P building rules: if you are in England/Wales then a new circuit (such as this) is notafiable work so it is generally easier and safer to get a professional electrician in to do that.