File sizes above 2GB or so have shown a consistent pattern of problems, so we recommend 2GB as a sensible "soft" maximum file size; files slightly above that size may still do okay, but the more your file exceeds that guideline, the more likely you will have problems.
Also consider whether most buyers would even have 3.4GB free on their hard drives to save the file, and how long you'd be expecting your buyers to babysit a download of that size, considering that any download can only be as fast as the slowest link in the chain, which is usually the buyer's own ISP connection speed:
http://www.numion.com/Calculators/Time.html
For sellers of extremely large files, we usually recommend taking advantage of our pre-integrated support for SwiftCD, so you could have them automatically duplicate your file(s) to a CD/DVD and mail that to buyers on-demand whenever an order comes in:
http://www.e-junkie.com/ej/help.integration.htm#swiftcd
