I don't believe we have the option any longer to select how many columns of tiles show. I think they just photoshopped that in from an old screen cap in the very beginning days of 1st gen Win HD. Note that that is 1 large, 2 medium or 4 small tiles across...
Now, by default we see the extra column of tiles due to having 720p and a 5" screen. Microsoft made it standard above a certain resolution and screen size. We have 1 large + 1 medium or 3 medium or 6 small tiles across and it is not user selectable.
The width of a medium tile is 1 column. Thus the largest tile can nly cover 2 columns out of 3.
So, you won't ever have one tile on 8.1 that covers the whole width of the screen. Sorry.
I'm afraid I don't know what App that is.