iMac 5K running windows 8.1

Hi

First time poster.

I just got a new iMac (5K) and I have installed fusion on it with windows 8.1.  Due to the resolution of the imac windows does not display correctly and doesnt take up the full screen when I go into full screen mode.  Am I doing something wrong and there are some settings I can change to fix this? or should I wait for an update to fusion?

thanks