Not so easy. Beamer has a lot of element each one with its font. If you look at the first page of http://www.cpt.univ-mrs.fr/~masson/latex/Beamer-appearance-cheat-sheet.pdf you can have an idea. You can also look at the file beamerfontthemedefault.sty somewhere in your distribution tree.
– RmanoMay 31 '21 at 07:41
https://tex.stackexchange.com/questions/56768/how-to-set-a-small-default-font-size-with-beamer could help
– RmanoMay 31 '21 at 08:17
beamerfontthemedefault.stysomewhere in your distribution tree. – Rmano May 31 '21 at 07:41