newbie q: staff size

[steve berthiaume]

hello; enjoying learning this program!

i want to enlarge the size of my trumpet chart, as we are all getting older and must apparently continue to play in poorly-lit venues :-(.

what's the easiest way to do this?  i've tried 

    \layout
   {#(set-global-staff-size 20)}

but this only generates an error:

   Warning: not in toplevel scope

and no change in the printout.

i've also tried
   \score
   {  .\layout{  #(layout-set-staff-size 15)  }}

in different permutations (with & without {} at various points etc.) but all this seems to do is increase the size of the music relative to the staff, so i get giant notes on a normal-sized staff.

thanks in advance for any help; here's my score so far (I can already hear everyone laughing!):

-steve

[begin code]
\version "2.10.20"

\score

  
{\relative c'   


%%%%%  Intro
  
    {     \set Score.markFormatter = #format-mark-box-barnumbers
        {\set Staff.instrumentName = "TPT"}
        
        \override MultiMeasureRest #'expand-limit = 1
        \set Score.skipBars = ##t R1*3
    
        r4 a'8.-> \(b16-- c4-- e--
        a2 e4.. a16--
        g4-- e2.\) \breathe
        e2---> \(d --->
        c--->\) r2 \bar "|:" \mark \default

%%%%%  Verse
        \set Score.skipBars = ##t R1*3

        a16-> \(<\parenthesize a>  a a-. ~ a16 a8-. a16 g8-- a-.\) r4

        \set Score.skipBars
 = ##t R1*2

        a16-> \(<\parenthesize a>  a a-. ~ a16 a8-. a16 g8-- a-.\) r4
        r1
        r4 b8.-> \(cis16 d4-- cis-.\)
        r4 a8.-> \(b16 c4-- b-.\)
        
        


    }

}
\layout
{#(set-global-staff-size 20)}


[end code]