Creating an error message

If you're writing a package, say foo.sty then

\AtBeginDocument{
  \if\@ptsize1\else
    \PackageError{foo}
      {Illegal font size option detected.\MessageBreak
       This is a fatal error, I'll stop the run}
      {You have specified a point size different from\MessageBreak
       the unique size this package supports, which is 11pt.\MessageBreak
       Correct your input file}
    \expandafter\@@end
  \fi}

will do.

If you're using one of the standard document classes (article, book or report), the macro \@ptsize contains 1 if you're using 11pt:

\documentclass[12pt]{article}
\makeatletter
\AtBeginDocument{%
  \newcommand{\@ptsizetest}{1}% Equivalent to 11pt for a document class option.
  \ifx\@ptsizetest\@ptsize\relax\else
    \@latex@error
      {You need to use 11pt font size}% Error message
      {Add this as a document class option [11pt]}% Help text
  \fi%
}
\makeatother
\begin{document}
Test document
\end{document}​

This creates the error

LaTeX Error: You need to use 11pt font size.

If you're using other packages that define the font size, some more work is required.