I just got a Dell XPS 13 2015 and I installed Ubuntu 15.04. Everything is working fine with the exception of using an external display.
The resolution on my laptop is 3200x1800, my external screen is 1920x1080. When I connect my laptop to my external display everything is twice the size.
I read this article https://wiki.archlinux.org/index.php/HiDPI#External_displays but it didn't help me to find a solution.