First make sure pse 12 is not running before doing the below steps.
Also, in windows 8.1, you need to have the Hidden Files and folders shown
http://www.ricksdailytips.com/view-hidden-files-and-folders-windows-8-1/
Go to
C:\Users\User Name\AppData\Roaming\Adobe\Photoshop Elements\12.0\Editor
and open the New Documents Sizes.psp in Notepad
Then after whatever number is actually in the file (two in my example)
paste this line and then File>Save
"785 px X 250 px" 785.00 250.00 px 72.00 dpi RGB 8 1.00 white "sRGB IEC61966-2.1"
Then start the pse 12 editor and the preset should show on the preset list in the new document dialog