The thing about smartphones, John, is that regardless of the screen size, they all (within reason) can do the same things. Where screen size comes into play is "comfort of use"...a larger screen is easier to read, especially for "more mature" folks but the trade-off is that a larger device is sometimes less convenient to carry around, at least in the manner that many folks are used to by simply using a pocket.
Decide what functionality you need first. Then work on deciding what device is best going to fit your needs for both size and capability.
--
The most expensive tool is the one you buy "cheaply" and often...