How do I split a .zip file into multiple segments?

You have existing.zip but want to split it into 50M sized parts.

zip existing.zip --out new.zip -s 50m

will create

new.zip
new.z01
new.z02
new.z03
....

To extract them, you should first collect the files together and run zip -F new.zip --out existing.zip or zip -s0 new.zip --out existing.zip, to recreate your existing.zip. Then you can simply unzip existing.zip.

You'd expect unzip new.zip would work, but unfortunately it's not implemented

warning [new.zip]:  zipfile claims to be last disk of a multi-part archive;
  attempting to process anyway, assuming all parts have been concatenated
  together in order.  Expect "errors" and warnings...true multi-part support
  doesn't exist yet (coming soon).

and in my tests, concatenating the parts as it suggests, i.e. with cat, and running unzip, failed to extract all my files.

This is what works for me:

zip -s 50m new.zip big.iso

For 50MG parts

new.zip
new.z01
new.z02
...

Create chuncks of 3G with this (good for puting large files on a FAT32 disk)

zip -s 3g new.zip big.iso

New Mac OSs extract these files when double clicking the new.zip file