The DPN holders use thick, durable elastic and wide rubber caps so that any thickness of knitting needle or project fit into them. The elastic stretches to fit over your needles without putting pressure on them, protecting the tips from catching on whatever else you're carrying around.
The blue 6" DPN holder is what most needles fit into. These hold sleeve or hat knitting needles
Please note that the above aren't the exact measurements of the DPN holder, they are instead the correct length to comfortably fit over that size double pointed needle.