The S7 is listed as having the same width and depth as the S6 counterpart, but the cases are 1mm taller for both versions (44/45mm refers to the case size, not the screen size).
I doubt S6 cases and screen protectors are going to work on S7 watches, outside of stretchy ones that can handle being pulled onto a 1mm taller case. But the new watches are also more rounded than the older models, so even those cases might not fit.