No not really a "max" as such, but you'll need to convert to base64 for the CSS so bigger file = bigger text string.
Best practice is to first resize to the resolution you want in CA PPM, then optimise the image for web.
Keep in mind bigger file equals reduced performance as the base64 image is reloaded everytime and not cached!
Update the Logo in Clarity PPM
How to change the logo in CA Clarity PPM v13 - YouTube
Also the logo appears in more than one place in the theme, i.e. login, header, web icon, gantt