I have just upgraded from Windows 8 to Win 8.1,and all is going well except for one issue. My start screen icons are much larger than they were in Windows 8, at the same resolution. The normal icons stack 5 vertically, and previously there were 6. I didn't change my DPI settings in Windows at all, this just occured after the update. I can live with it, but it ends up causing more scrolling because the icons are larger.
I am at 1920X1080 resolution on an ATI 6850M. I have the DPI settings in Windows set to medium - but it was that way in Win 8, and changing it has no effect on the start screen. I also tried the new scaling option for modern apps, but it had no effect on the start screen.