I don't know how can I bring iPad in xcode preview? Right now It is missing. I have checked that all simulator is listed in
xcode > Windows > Devices
Version 7.3.1 (7D1014)
For the getting list of all devices size preview, first you need Enabling
Size Classes in Interface Builder. As per following screenshot you just need to check right mark in
After enabling size classes now check the preview. You get the full list of all devices.
sizeClass that wont appear the iPad list
Let me show you real time GIF: