On the web, people came up with the concept of “mobile-first”, meaning that you should first of all make sure everything can work with the smallest size, and then add whatever adjustments, improvements, extra functionality due to size, <i>&c.</i> for larger screens after that.<p>(Of course, in practice people made a hash of it (as they always do) and it often became mobile-<i>only</i> design, with users of larger screens getting a comically bad experience that was manifestly designed for tiny screens; but that’s not the idea of mobile-first.)<p>I’d say it’s generally a sound philosophy, even though at larger sizes you may desire to recompose the UI quite substantially. It’s generally easier and/or safer to design for small screens and reformat or add extras for larger screens, rather than to design for large screens and reformat or remove for smaller screens.<p>But people definitely regularly don’t go small enough in their idea of “mobile”. I decided some years ago that I would target 300px as my baseline width, and I feel that’s served me well. (And if convenient, I make it work past 260px.) 300px is a bit smaller than <i>anything</i> mainstream, and is thus pretty safe. (I haven’t often done much that needs a baseline <i>height</i>, but I’d use 300px and/or 450px there, considering both portrait and landscape usage as necessary or applicable.)