I think that in the "big file" case I'd stick to the ZIP splitting feature instead of rolling my own. My only concern is that in this case you have to be sure that the recipient is sufficiently smart to save the attachments (instead of dumbly double-clicking on them) in the same directory, and launch the main zip file (but this is simpler, it's the only zip file!).

The mail-an-URL solution is also my preferred one, when feasable. But sometimes it's simply not possible: in the particular case that started the development of my small script, we were into a tender and the rules were that the stuff had to be both sent by email and delivered via CD-ROM.

Thank you for the suggestions, anyway!