Hide menu item or widget on desktop

Hey there!
I have a login widget in my main header menu that I only want to be visible on mobile versions (tablets and phones) or in triggered menu. I have idea to use media queries. But I don't know what the breakpoint is for my theme ("Tesseract") (what value to use e.g min width: 480px). My site is here (m52study.tk) please have a look and advise.
Thanks for your assistance.
Have a nice day,