You can change the two line after the one you mentioned:
\def\@rticle@options{10pt,twoside}
% fleqn
\DeclareOption{fleqn}{\def\@rticle@options{10pt,twoside,fleqn}%
Replace the "10pt". But don't change the original class but save the file under a new name.
Edit: svmono not only loads article.cls but also change some font sizes later on. Search the line
% fonts, sizes, and the like
and correct the definitions which follows. You can use the values used in size12.clo as template/inspiration.
Btw: This is a very special class. Why are you using it instead of a class e.g. from the KOMA bundle or memoir?
svmono.cls has some hard-coded design choices; one of them is the use of 10pt settings. One way to acces 12pt settings is to make a copy of svmono.cls, rename it as mysvmono.cls, for example, and make the necessary changes in the copy.
To use the 12pt settings one can carefully use the definitions used in another class, for example article (which svmono is built upon); the necessary definitions are in the file size12.clo (under .../text/latex/base).
The lines that will need a change are
% no size changing allowed
\DeclareOption{11pt}{\OptionNotUsed}
\DeclareOption{12pt}{\OptionNotUsed}
% options for the article class
\def\@rticle@options{10pt,twoside}
% fleqn
\DeclareOption{fleqn}{\def\@rticle@options{10pt,twoside,fleqn}%
which should be replaced with
% no size changing allowed
%\DeclareOption{11pt}{\OptionNotUsed}
%\DeclareOption{12pt}{\OptionNotUsed}
% options for the article class
\def\@rticle@options{12pt,twoside}
% fleqn
\DeclareOption{fleqn}{\def\@rticle@options{12pt,twoside,fleqn}%
And the section under % fonts, sizes, and the like:
% fonts, sizes, and the like
\renewcommand\normalsize{%
\@setfontsize\normalsize\@xpt\@xiipt
\abovedisplayskip 10\p@ % \@plus2\p@ \@minus5\p@
\abovedisplayshortskip \z@ % \@plus3\p@
\belowdisplayshortskip 6\p@ %\@plus3\p@ \@minus3\p@
\belowdisplayskip \abovedisplayskip
\let\@listi\@listI}
\normalsize
\renewcommand\small{%
\@setfontsize\small{8.5}{10}%
\abovedisplayskip 8.5\p@ % \@plus3\p@ \@minus4\p@
\abovedisplayshortskip \z@ %\@plus2\p@
\belowdisplayshortskip 4\p@ %\@plus2\p@ \@minus2\p@
\def\@listi{\leftmargin\leftmargini
\parsep \z@ \@plus\p@ \@minus\p@
\topsep 6\p@ \@plus2\p@ \@minus4\p@
\itemsep\z@}%
\belowdisplayskip \abovedisplayskip
}
%
\let\footnotesize=\small
%
\renewcommand\Large{\@setfontsize\large{14}{16}}
\newcommand\LArge{\@setfontsize\Large{16}{18}}
\renewcommand\LARGE{\@setfontsize\LARGE{18}{20}}
%
which should be changed to
% fonts, sizes, and the like
% change to 12pt (GM: several lines)----start
\renewcommand\normalsize{%
\@setfontsize\normalsize\@xiipt{14.5}%
\abovedisplayskip 12\p@ \@plus3\p@ \@minus7\p@
\abovedisplayshortskip \z@ \@plus3\p@
\belowdisplayshortskip 6.5\p@ \@plus3.5\p@ \@minus3\p@
\belowdisplayskip \abovedisplayskip
\let\@listi\@listI}
\normalsize
\renewcommand\small{%
\@setfontsize\small\@xipt{13.6}%
\abovedisplayskip 11\p@ \@plus3\p@ \@minus6\p@
\abovedisplayshortskip \z@ \@plus3\p@
\belowdisplayshortskip 6.5\p@ \@plus3.5\p@ \@minus3\p@
\def\@listi{\leftmargin\leftmargini
\topsep 9\p@ \@plus3\p@ \@minus5\p@
\parsep 4.5\p@ \@plus2\p@ \@minus\p@
\itemsep \parsep}%
\belowdisplayskip \abovedisplayskip
}
\renewcommand\footnotesize{%
\@setfontsize\footnotesize\@xpt\@xiipt
\abovedisplayskip 10\p@ \@plus2\p@ \@minus5\p@
\abovedisplayshortskip \z@ \@plus3\p@
\belowdisplayshortskip 6\p@ \@plus3\p@ \@minus3\p@
\def\@listi{\leftmargin\leftmargini
\topsep 6\p@ \@plus2\p@ \@minus2\p@
\parsep 3\p@ \@plus2\p@ \@minus\p@
\itemsep \parsep}%
\belowdisplayskip \abovedisplayskip
}
\renewcommand\scriptsize{\@setfontsize\scriptsize\@viiipt{9.5}}
\renewcommand\tiny{\@setfontsize\tiny\@vipt\@viipt}
\renewcommand\large{\@setfontsize\large\@xivpt{18}}
\renewcommand\Large{\@setfontsize\Large\@xviipt{22}}
\renewcommand\LARGE{\@setfontsize\LARGE\@xxpt{25}}
\renewcommand\huge{\@setfontsize\huge\@xxvpt{30}}
\let\Huge=\huge
\newcommand\LArge{\@setfontsize\Large{18}{23.5}}
%
This new file must be saved in the local TDS directory, or somewhere else safe where TeX can find it.
The complete file mysvmono.cls with the modifications mentioned was too long to put it here directly.
Best Answer
The
svjour3.clsdoesn't define any other font option than10pt.So there's no real way to override the decision made by the class developers. You might add
paying the price of an annoying warning and risking bad output in some situations.