I'm not aware of any way to change the tab font size.
If reading the names of the tabs is an issue, you may want to organize the
number of tabs so that the tabs never scroll horizontally.
If there is enough room on the screen to display all of the tabs in a given
folder, then the tab width changes to display the entire title. Once more
tabs are present than can be displayed at once, the tabs shrink to a uniform
size. Of course, that may not be possible or convenient given how you have
your Notebook organized.