Here I found something to change the default label width for the whole document (note that I changed ’lyxlist’ to ’labeling’):

%adjust the labelwidth in the LyXlist environment
\newlength\extrawidth
\setlength\extrawidth{1eM} % default value

\renewenvironment{labeling}[1]
{\begin{list}{}
{\settowidth{\labelwidth}{#1}
\addtolength{\labelwidth}{\extrawidth}
\setlength{\leftmargin}{\labelwidth}
\addtolength{\leftmargin}{\labelsep}
\renewcommand{\makelabel}[1]{##1\hfil}}}
{\end{list}}