On starting the El Capitan install from a running Yosemite OS an error dialog may appear complaining that there isn't enough free space on the startup disk to proceed.
After seeing this i erased some large files then tried again. This time the install continued to the point where, after reboot, it tried to proceed for about 30 minutes before it gave up. Once again it complained that there wasn't enough free disk space. Apparently it under-estimates the amount it needs on starting the install.
The install doesn't cleanup on failure and allow Yosemite to boot and it can't get any further without freeing up some disk space. To resolve it:
power-off the machine
restart the box. On hearing the startup sound, press cmd-r to boot it in recovery mode
in recovery mode, use the menu to select the 'terminal' option
in the terminal run the following commands:
diskutil list
to get a list of the partitions and their sizes. Hopefully the sizes allow you to determine which partition is the problem.
diskutil mount /dev/disk0s1
where disk0s1 is the problem partition. It may be different in your case. If you aren't sure, you may have to repeat this process until you mount the correct one.
cd /Volumes/Untitled\ 1
where "Untitled\ 1" is the label specified during creation of the disk. It may be different in your case. If you mount then unmount the directory a few times you will be able to tell which directory is being used.
once you have the correct root directory you can cd to, say /users/username/Downloads then remove any large files using the terminal 'rm filename' command. It's best to be careful - you can't easily undo file removal here.
after cleaning up any large files:
cd /
diskutil unmount /dev/disk???
Use the recovery menu to reboot. If you have removed enough file the El Capitan install should now have enough disk space to continue.
Hope it helps.