I have just come across this issue and it is incredibly annoying.
I am using Windows 8 Pro 64 bit, with Office 2013, on an HP Revolve 810 hybrid laptop/tablet.
Every time I open an e-mail, or open a new mail window it comes up full screen. No matter what I try, I cannot get Outlook to recognise the fact I don't want them open full screen.
I am using the native resolution for this machine and have not increased the dpi settings at all, it is set for 100%.
I have used Outlook 2013 on a number of different machines before, but this is the first time on a hybrid/tablet and it's the first time I've seen the issue.