I have two monitors on my Windows 7 x64 machine:
Primary monitor: Resolution 1920x1200, with the Windows taskbar positioned vertically along the left side of this screen.
Secondary monitor: Resolution 1600x1200, positioned to the right of the primary monitor.
If I try to make mRemoteNG go Full Screen on either of these monitors, I encounter problems.
On the secondary monitor, running 1600x1200, mRemoteNG becomes some size that is bigger than the actual resolution on that monitor. Perhaps it's resizing to the same size that would be necessary on the Primary monitor. As a result, several hundred pixels of the entire application window get chopped off. In my case, for example, I normally have the Connections and Config panels on the right side, and when I go Full Screen on this secondary monitor, these disappear (in addition to things like the Panel controls and Tab controls which get chopped off too).
On the primary monitor, mRemoteNG sometimes goes Full Screen in such a way that initially it goes behind the Windows taskbar. In other words, it becomes the full 1920x1200 in size, meaning a vertical strip of the entire application is obscured by the taskbar. Clicking anywhere again in mRemoteNG is then necessary to bring it to the foreground, in front of the taskbar. Reproducing this seems to depend on exactly what has focus within mRemoteNG at the exact time you attempt to Full Screen. I can most reliably reproduce this when the menu bar has focus. So clicking on the View menu and then Full Screen should demonstrate the problem.
I'm not using any special multi-monitor software.