Change font size of \Large etc.

The size for article.cls with option 11pt are declared in size11.clo. You can copy the definitions and change them. E.g. the original \Large is setup like this:

\newcommand\Large{\@setfontsize\Large\@xivpt{18}}

So use in your preamble (or a local sty).

\makeatletter %only needed in preamble
\renewcommand\Large{\@setfontsize\Large{13pt}{18}}
\makeatother

The 18 is the baselineskip (in pt). Adjust it to the value you want to use.