how to set staff size in safe mode

[Ed Stauff]

How can I set the staff size in safe mode?  I've only been able to find 
two ways in the documentation: set-global-staff-size and 
layout-set-staff-size, and neither one works in safe mode.  I get this 
error:

3.ly:8:12: error: GUILE signaled an error for the expression beginning here
\layout {  #
            (layout-set-staff-size 15) \context { \Score \remove 
"Bar_number_engraver" } }
Unbound variable: layout-set-staff-size

I'm using 2.10.20.

Thanks,

-- Ed