If you have installed the new Windows Phone 8.1 SDK and having problem with some of the emulators your are probably not alone. This message appears for emulator 4.7 and 5.5 inch:
The specified diagonal screen size is not valid. (Added for search engines to find this post)
This problem occurs for us developers which are from countries where we use a comma for decimal symbol instead of a period. All emulators which have a period in their size (4.7 inch and 5.5 inch) will not work unless you change this in the Control Panel – Region setting.
Just change the Decimal symbol to a period instead of a comma and the emulators will work again. What this has for effect in your other programs on your computer I can’t tell but if you really want to run the 4.7 or 5.5 emulator this is the way to do it.
Perhaps sometime in the future the US will learn that they are not alone in the world and remember to try their new features on computers with other settings.