I know some fonts aren't able to change size. But in this case, ALL fonts remain at their default setting and the font size drop-down to the right of the font name is greyed out, unable to be edited.
I'm using Catalina on a 2019 MacBook Pro.
MacBook Pro 13″, macOS 10.15
Have you tried clicking in the size dropdown and manually entering a number then press return?
I've had it where it won't let me select a font size from the dropdown, but if you manually enter it in the dropdown it seems to work for some reason.
Hope that helps.
Try deleting preferences. That procedure often cures glitches like you are experiencing.
To delete preferences, open iMovie while holding down the Option and Command keys and select to delete
preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.
Now see if you can change font sizes.
-- Rich
Unable to change font size in iMovie titling
