Thread:Extension talk:Score/how to set size/reply

Looks like safe mode disables that. From the manual: The -dsafe option works by evaluating in-line Scheme expressions in a special safe module. This safe module is derived from GUILE ‘safe-r5rs’ module, but adds a number of functions of the LilyPond API. These functions are listed in ‘scm/safe-lily.scm’. In addition, safe mode disallows \include directives and disables the use of backslashes in TeX strings. In safe mode, it is not possible to import LilyPond variables into Scheme.