When you define an enclosure in an RSS feed, you have to specify a length attribute, which specifies the size in bytes of the file attached in the enclosure.

This is actually quite simple: the Finder’s “Get Info” window not only shows how many KB, MB, or GB the file takes up on disk, but also what this is in bytes (this info can be found in “General”).

Simply take out the commas, and there’s the enclosure’s length!