36" window in a 51" wall--why not?

[hoodatiz]

I have the center wall of this bow wall assembly-almost like half a gazebo which is 52-11/16 wide. Into it I have put a std size casement which is 36-3/16 wide. I'm putting an broken arch transom over it and am trying to copy it into the four adjacent walls which measure 51" wide and all Chief will let me paste is a 35" unit, automatically narrowing it for me. NASA is not on my resume, but I gotta think I can get a 36-3/16 wide unit in a 51" wall. What obvious thing am I missing??