There are many reports available in Google Analytics which can be downright misleading. One is the screen resolution of the devices which your visitors use to access the web. If you look at this data, you’ll almost certainly see a mixed pattern of screens, with those big old 1920- or even 2560-pixel-wide screens on the increase. Then again, there’ll also be an increase in tiny 320-pixel-wide screens too, as cheap smartphone access gains its share of views.
So this should give you a good idea of the screen sizes which you should be designing your site for, right?
Unfortunately, you’re looking at the wrong data. It’s not about the screen size which people are using, it’s the window size which matters. A smartphone browser window will take up the whole of its screen, but on a big 27-inch desktop display, most users will be viewing web pages in smaller windows. And it’s the size of those windows which matters.
As often happens, Google Analytics gives you the information you need, but it’s just not obvious where to look. The ‘Browser Size’ data turns up as a ‘secondary dimension’ in standard reports, or you can get it by making a custom report. This is a good article on the sort of thing you can do.
Your web page design needs to have certain things in view without scrolling. Investigating the browser sizes most commonly used to view your site is a good way to ensure that happens.