What is the maximum screen size for a phone?

Question

css

I want to set up media so that when you open the site from your phone, another css file is connected. The idea is as old as the world, in fact. But at what value of width should this be done?

Answer 1

Once there was this one:
But manufacturers do not stop, bigger diagonals appear:
And even more!
https://www.youtube.com/watch?v=fXCETvEce3w
And if you are not talking about the physical diagonal, but about the resolution, then 4k phones seem to be already on sale.
Don't bother with the phone/non-phone separation. Look at the size of the viewport, or rather the size of the window: www.quirksmode.org/mobile/viewports.html

Answer 2

There is no maximum "phone" screen size. Your resource can be opened from a tablet with a resolution of 2560*1440. And here the question arises, can you "just right" unmistakably separate the tablet and phone?
My advice is to plan your prototype, design, and responsive layout, don't pay attention to the phone/tablet/PC division, but adjust to the size. You need to adjust only when writing styles for browsers for "phones / tablets / PCs".

Answer 3

For the site to work adequately, you need to determine the following parameters:
1) whether the device has a touch input (to determine the minimum size of the active element, and when to enable / disable hover-behavior)
2) what dpi (ppi) the device has (can be determined with using media)
3) what is the viewport resolution of the device. Maybe a 4k display, and the viewport will be set by the developer as 384px by 968px
And most importantly: in adaptive design, the device is not important, it is important to adjust the design to the type of input (touch - mouse) and to the viewport. Present the client not as a phone / tablet / laptop, but as a stretchable screen from 4 to 27 inches. Less than 4 - a completely different UI (all kinds of clocks), more than 27 - TVs have already gone, also with their own quirks. There, the capabilities of the browser are limited and the type of input is different - 3d-mouse or magic-mouse