the width of the label isn't defined in the
.bst file, but (usually) in the document class or bib style package (e.g.
natbib.sty). look in the
.sty file that you're using for the
thebibliography environment. the width of the indent will be set with
\leftmargin, but the exact components will vary depending on the particular style being used.
in all styles i'm familiar with, this setting is done within a
\list within the definition of
thebibliography, so any resetting will depend on either a redefinition of the entire environment, or (if you're lucky) of a subcomponent that can be redefined separately.