Hi,
It must depend on the physical size only, this is for usability purpose.
For instance you could have a Full-HD device on a 5" screen, in this case the user interface will have to adapt to the physical size otherwise this wouldn’t be usable with adult fingers.
In the actual state of Cubase iC Pro’s user interface, our usability tests have shown that it is no longer possible to have a usable split-screen interface when the screen size is below 7.9".
Another way to look at it is, how many people have 7" smartphones, seriously?
Internally we rather call them the small and the large versions because it’s not related whether it runs on a phone or a tablet strictly speaking.
Cheers,