While branding a mobile first startup I have come to realise that a set pixel height/width need not be the best answer for figuring minimum size, Different phones seem to have different pixel density and hence the logo at particular minimum size might appear crisp on one but illegible on another. Anyone has an answer to this?
It won’t be illegible, but the size can differ. Well, unless you count sharp but too small to read as illegible. Then again, in those instances where it will differ, it will differ for every other .jpg or png on the site too. Really though, the manufacturers of these devices (including larger displays) do a pretty good job compensating for these pixel density differences, so it’s not something to be overly concerned with.
The other approach — often better anyway — is to use .svg instead of .png.
That’s the solution.
I agree with @Just-B
Also, what would be the best way to test minimum pixel size of the logo? @Just-B
Make sure your signature image no larger than 320px wide, and 70–100px high. Most mobile devices are typically between 320px and 500px wide, so this will ensure your logo looks great on all mobile screens !