Q: How can I reduce space on the startup disc to enable the download of Mavericks?

  I have enough overall disc space but the update won't download and tells me to reduce "startup" disc space.

How much free space do you have?

This warning usually indicates, that your free disk space is critically low.

Empty you Trash, delete files that you do not need at all, and move large media files, like iPhoto libraries or videos to an external drive.