Basic approach would be similar to gravel sorting.
Start with a trough of some sort that allows only one round of most likely sizes involved to slide down one round at a time.
Cut a slot just long and wide enough for the smallest caliber to fall through, which will not be the actual case measurement possibly a bit smaller, further down the trough next largest size and so one.
Have a bucket under each slot so that indicates how far apart the slots should be.
Feeding the brass into the trough and keeping it moving is likely a balancing act.
Or accept some similarity of size to reduce complexity such as with
https://www.badmanbullets.com/OnlineStore/proddetail.php?prod=Reloading-Ammo-Brass-Sorter
or similar sifters
Or find someone with one of these and pay them to sort it:
https://www.youtube.com/watch?v=fcfE49ImaSU