I'd say that option 1 is best. It's possible to tweak the outer dimensions. Most time you'd want it to have the same size as the phone, but lets say you want to add two front facing speakers at the side of the screen.

The inner dimension will not change. It has to fit the phone and the phone stays the same. If Jolla releases something else (like a BIG tablet) that have other dimensions, lot of other things will also change and the outer dimensions wont stay the same anway.