Changing font size instruments in score

If you mean the names of the instruments, it's a bit awkward, but possible. Make the desired change to Style / Edit Text Style - both long and short instrument names. Then go through your score instrument by instrument, deleting the old names and retyping them.

Obviously not ideal, but it's the best way I know to fix an existing score if you've changed scaling dramatically after score creation so your instrument names are not sized well any more. For 2.0, style changes will automatically update existing text, so things get much better.

Meanwhile, though, the lesson is, don't change score size dramatically after score creation if you can help it. Use a template (perhaps one you create yourself) that has sizes the way you want in the first place. Another possibility - create your score with only a single dummy instrument, size things the way you want, then add the instruments you really want and delete the original dummy instrument.