Birthday problem/attack, part $n$

Previously:
https://yurichev.org/birthday2/
https://yurichev.org/birthday/
https://yurichev.com/n/birthday/

Another thing many people observe.
Pastebin-like services, where an URL contains hash of message/file.
A message/file hashed using MD5, SHA, etc, and hash maybe encoded in base64 or whatever.

( See also my post about content-addressable storage: https://yurichev.com/blog/CAS/ )

To make URLs short, you may want to cut result of SHA2-256, let's say.
But not too short.
But how short?
How to determine the right length?
It's easy --- depending on number of messages.
Say, a 32-bit identificator can address ~2^15 messages (almost) without collisions (roughly).
A 48-bit identificator can address ~2^23 messages...
A 64-bit identificator can address ~2^31 messages... etc.