As a developer and not a designer, I like to have my design on iPhone6 size.
About the views,they should be scalable in some way. You have to make sure it's either scrollable up&down, so smaller devices can see all the content, or if it's not the case, you should have some kind of scalable view inside that will shrink if needed. Most of the time, my views are scrollable and content doesn't change size. If they contain a scrollable view, then the view itself does not scroll, but I shrink/expand the scrollable view. For example, if I have a list in the middle, on the iphone 6 it will display 4 or 5 cells, while on a 4S only one and a half. Btw, always show the next half to show the user he can scroll. Anyway, the the iphone5 resolution problem is not really a problem.
Concerning the assets, if they are made of vectors then they scale to any size, otherwise I like them in @3x and reduce them myself using the sketch export function. That way, I can have all my assets on the highest resolution and reduce if needed.
And I think that's all there is to it, but again, that's from a developers point of view.