Use menu item: Format -> Style -> Text Styles -> Lyrics (you can select odd / even). Then change the font size.
-OR-
Right click on a single lyric -> Select -> All Similar Elements, and then change the text size in the Inspector.
-OR-
Left click on a single lyric, change the text size in the Inspector, then press the 'S' button to 'set as style'.
Comments
Format/Style->Text Styles->Lyrics(Odd/Even lines)->set size.
Or right click a syllable->All similar elements->Inspector set size.
Use menu item: Format -> Style -> Text Styles -> Lyrics (you can select odd / even). Then change the font size.
-OR-
Right click on a single lyric -> Select -> All Similar Elements, and then change the text size in the Inspector.
-OR-
Left click on a single lyric, change the text size in the Inspector, then press the 'S' button to 'set as style'.
For Inspector info., see:
https://musescore.org/en/handbook/3/inspector