Check in the View menu to see what the Keyboard shortcut is for full screen. I am not in front of a PC right now so can't verify if it is indeed F11.
After doing some searching I came across a thread on the Phase One User Forum that raises the same issue of not being able to get a full screen image of a photo in C1 on a Windows O/S. See this link for details
https://forum.phaseone.com/En/viewtopic.php?f=69&t=26440