I am running Windows 10 Professional in 1920 x 1080 resolution. I have two (2) monitors. I can resize the Start Menu vertically but not horizontally on either monitor. I have plenty of horizontal and vertical screen real estate, as can be seen by this screen shot of my desktop:
But every time I grab the right side of the Start Menu and attempt to drag it to the right (i.e. to widen it), it doesn't work.
What am I missing here?