> Unfortunately the curly brace created for these four instruments is way too wide. Is there a way to shrink the vertical size of the curly brace for the GrandStaff (so that it would have the same width as the curly brace of a GrandStaff consisting of only two instruments)?
Another solution would be to scale the brace like so:
\override GrandStaff.SystemStartBrace #'stencil = #(lambda (grob)
(ly:stencil-scale (ly:system-start-delimiter::print grob) 0.75 1))
Here the brace is scaled horizontally to 0.75.
-David