A good design process accounts for any screen size. Not breakpoint-based-design, but true responsive design. However, I have to admit that for the past year all of my artboards in Sketch were sized for the iPhone 6. Screens have been trending larger and larger for nearly a decade, so it was relatively safe bet to anticipate the extra screen real estate.
After today's announcement, I'll be switching back to a smaller default artboard that will be more representative of a 4-inch screen.
Not only does this make sense from a user experience perspective (hey, your design will be optimized for emerging markets!), but also it makes sense from a practical process point of view. Scaling a design up one inch is infinitely easier than scaling down by the same amount.