It's possibly a personal choice Julie.
I haven't seen anywhere a specified figure, so I work with a max file of around 1 mb which most email systems can handle and will fit on a floppy disk if anyone wishes to transfer it using that medium. It's possibly better to err on the side of smaller rather than larger given that you don't know what type of systems your users have. Even if you deliver on CD, Read Only Memory and Video RAM can present problems if files are too large.
Let me know if you find a more definitive answer.
Desert Wave Enterprises