They have removed this feature in CC 2015. Please see: Extract assets
Here is the replacement: Photoshop Help | Export artboards, layers, and more
I hope this information helps.
Edit: If you want to continue using the Extract feature you can role back to 2014. See here: Download and install Adobe Creative Cloud apps
The next update of Photoshop will bring back an option in the Export workflows to output different sizes for your assets.