It will be creating an iso image in /tmp/
but you probably have space in tmp than the size of the 'project'
as people have said, df -h ( open a terminal and type it )
will tell you how much space, you have not got, in /tmp
What they are looking for is where you have space available
So, look in the column 'Avail' for something greater than 9 GiB ( or just 5 gig if single layer )
Settings --> configure k3b --> misc --> Default Temp.. dir.
Change that to a path that has enough space.
Alternatively, when it comes to Copying the Disk, click the 'image' Tab and navigate to somewhere with enough space.
If you have *lots* of ram then /run/shm/ is a good place, will be nice and fast
Just hope k3b cleans up after itself.