Broken longrightarrow

If you want to decrease the size of the symbol produced by longrightarrow, you should either preface the command by small, footnotesize, etc before entering math mode or employ scriptstyle and scriptscriptstyle directives inside math mode.

Actually, judging by this screenshot, it may be a good idea not to use tiny or scriptscriptstyle together with longrightarrow...

documentclass{article}
begin{document}
obeylines % just for this example
$longrightarrow$ 
{small $longrightarrow$} 
{footnotesize $longrightarrow$}
{scriptsize $longrightarrow$}
$scriptstylelongrightarrow$
{tiny $longrightarrow$}
$scriptscriptstylelongrightarrow$
end{document}