When writing chamber pieces including piano, I commonly send my pianist a version of the score with a reduced staff size in the other instruments. However, I don’t know of any way to create multiple layouts so as to maintain one full score layout with regular staff size and one with modified sizes. Currently, if you create a second layout of the full score, any engraving changes apply to both layouts. As it stands, I have to redo most engraving every time I make revisions and need to export the full score along with a piano score.
The only workaround I can think of is to create two files of the project, but this runs the problem of having to transfer over any edits from one to the other.
I think this is what you are after. Go to setup mode. On the right side duplicate the full score. Give the copy a new name. Go to that layout. (I use the pull down menu at the top of the screen). In write mode, select the first measure of the instrument you want to change. Right click and go to the staff size option. Select the size you want. When I went back to the Full Score layout, the instrument I had reduced in the duplicate layout was the original 100% size.