According to LyX Wiki, I can define my font size by means of this code (in case of 10.5pt) :
\usepackage{type1cm}
\renewcommand\normalsize{%
\@setfontsize\normalsize{10.5pt}{12pt}
\abovedisplayskip 10\p@ \@plus2\p@ \@minus5\p@
\abovedisplayshortskip \z@ \@plus3\p@
\belowdisplayshortskip 6\p@ \@plus3\p@ \@minus3\p@
\belowdisplayskip \abovedisplayskip
\let\@listi\@listI}\normalsize
But how can I calculate this value in case of 14pt, 16pt, 18pt?