Unfortunately, you're going to create a separate design for the iPhone 7+, as we don't have a zoom to fit feature for mobile. In general, you'll want to modify your design anyway, as the proportions for the artboard are slightly different than for the other form factors.
Thanks for trying to clarify, Elaine! Although I must not be the only one who has to develop for a different form factor than the iPhone I have on hand...I am well aware that the iPhone 7 Plus and its 1920x1080 screen is still a comparatively small minority amongst all smartphone users, but that's what I have to preview on...and although I do know that the aspect ratios are different between iPhone 5 and 6/7+, it would be very nice to "only" see the preview with a letterbox or pillarbox rather than a thick black frame all around (at least as an option).
I know your to-do list must be epic and it doesn't help that every developer bombards you with his/her own requests, so rest assured that I do understand if you can't meet this particular request any time soon, but I did want to throw my hat in the ring for a preference setting to allow up/downscaling of the preview when selected.
our programmer is well aware and highly capable of implementing my design in such a way that it will respond to the various aspect ratios of the great majority of the devices we're developing for...fortunately the aspect ratios don't deviate so greatly that it will pose a great problem.
thanks for your help, Elaine, and thanks for providing us with such an amazing tool...can't wait to see it flourish into something as mature as the rest of the Creative Suite!