For giggles... I logged into EQ directly, without ISBoxer, and viola, it is full screen windowed mode edge to edge, like I always wanted. So long story short, ISBOXER not required for a pretty UI.
Doing an alien autopsy on my eqclient.ini - here is what I found relevant, your mileage may vary.
WindowedModeXOffset=-8 ;Adjust per your Window's theme border width
WindowedModeYOffset=-31 ;Adjust per your Window's theme title bar height
RestoredWidth=1920 ;Set to the full resolution of your monitor
RestoredHeight=1080 ;Set to the full resolution of your monitor
AlwaysOnTop=1 ; hides the window's taskbar, hit windows key to bring it back.