I've seen this question asked multiple times over several years yet the issue persists. I'm using verison 18.104.22.168 (Arch Linux), but have been a user since the StarOffice days. Posters in Ask LibreOffice have been told to change their system font or modify a gtkrc file, to build some complex stylesheet, or to scale up the entire spreadsheet, which in the current version has no impact on the font size of sheet names.
The only references I can find similar to this are from 2011 and 2012 (Bug IDs 36772 and 46357).
If every cell in a spreadsheet can have a font size attribute, why not the sheet name? I see there are other attributes I can toggle. Personally I'm at wits end on this as my vision requires I both wear reading glasses and scale up the sheet. I'm about to search alternate software solutions. Please give real consideration to opening this as a valid LibreOffice (Calc) bug.
UI vs content, we can scale content with Zoom. Menus, sheet names, formula bar input get fixed assignments with UI--and os/DE controls scaling of the UI.
We don't, can't control the individual UI elements.
*** This bug has been marked as a duplicate of bug 101646 ***