MacOS – Automatically zip attachments on Mail getting sent

Use the -s switch on the zip command in terminal. So if your folder was called FolderName

zip -r -s 64 archive.zip FolderName/

64 is the size of the split (in this case 64Mb).

Use -s to set the split size and create a split archive.  
The size is  given  as  a  number  followed optionally  by  one  
of  k (kB), m (MB), g (GB), or t (TB) (the default is m). [1]

Use zipsplit to split a zip file in to multiple smaller zipfiles. Use the -n switch to set the size of the splits.

[1] http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man1/zip.1.html

As its man page states,

The compress utility reduces the size of files using adaptive Lempel-Ziv coding.

In contrast, the zip command (also present in Mac OS X, at least in Mountain Lion) uses a different algorithm. If a publisher is expecting a zip-compressed file, a file compressed with compress is something different, and will not work for this.