On Linux, you can customize it as needed by creating a custom stylesheet for Qt. More details can be found here.
There's an answer to a similar question here: https://mathematica.stackexchange.com/questions/95962/how-to-increase-the-font-size-used-in-mathematica-menus
I don't know anything about Linux menus or how Mathematica interacts with them.
There's an answer to your question (or a similar question) about Linux menu size posted here: https://community.wolfram.com/groups/-/m/t/1266184 . I'm not a Linux user. If that solves your problem, please add a follow-up message here saying what you did. HTH.
