I noticed on my Surface RT that they keyboard shrunk when I added a 4th row of icons adding:
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Scaling]
“MonitorSize”=”12.1″
and then going Settings -> Tiles -> Show More Tiles
I turned off show more tiles, and they keyboard was still small. Then, I deleted the registry entry (restoring it, essentially, to the way it was before), and the keyboard came back to its proper size. I haven't experimented, but I suspect that if you try different sizes with the entry, the keyboard size will change (smaller number should be bigger touch keyboard).
12.1 is a string.