I don't think this would make much sense. If you're up for a secondary display, I don't see 7" as a reasonable size. You're living at a size that makes a secondary screen somewhat superfluous there, depending on the size of that screen (I'd say anything under 27").
I've supported this idea from the get-go, in a way. I've been saying that I wanted the Surface Phone to be an alternative to the ASUS ZenPhone concept. With Continuum, you have the ability to sent up a Surface Pro chassis in the way we saw with that Continuum laptop deal. You could have a gutted Surface Pro (perhaps filled with a massive battery to charge the phone at the same time) where the phone slides and locks into the back via Type-C.
I've got fairly large hands, though not massive. I'm sitting next to a 5.2" Lumia 950 that is about as big a phone as I would want. 7" would just not make enough sense. I think you want 5-6", and I've just basically sat in the middle and called my goal 5.5". I'd really like to see them cut back on the vertical bezel regardless, but that's not a big deal. A 7" display just isn't portable "enough" to be a phone for many people. Given that the Surface Phone will be looking at a niche audience by OS interest from the start, throwing it into a highly niche form factor market to boot is asking for trouble with sales. I'd say the aim for optimal sales potential is 4.5-5.5", but as I said before, I've expected the range to be a bit higher than that.