I'm not sure if I understand your question, but I have a feeling that you're looking to typeset a part of the section header smaller than its surroundings. However, if you are in fact looking to force some characters to be typeset in uppercase or lowercase, do see @GonzaloMedina's answer for a demonstration of the use of the command \MakeLowercase (and its twin, \MakeUppercase).
By the way, the command \small does not take an argument but affects all subsequent material until the end of the "group" that contains the \small command is reached. Thus, don't write \small{rs} but, rather, {\small rs}. Because the relative font size used in section headings is \Large, which is two steps up from \normalsize, and because \small is one step smaller than \normalsize, use of the command \small results in text in the section header that's three (relative) steps smaller than the surroundings.
If you're looking to reduce the font size by a number of steps without needing to know what the absolute font size currently in use may be, you could use the relsize package and its \smaller and \relsize{-2} commands. The following MWE shows how this may be done.
\documentclass{article}
\usepackage{relsize}
\begin{document}
\section*{Writing and Reading using rsEGFPs} % no change in size
\section*{Writing and Reading using {\smaller rs}EGFPs} % 1 step smaller
\section*{Writing and Reading using {\relsize{-2}rs}EGFP's} % 2 steps smaller
\section*{Writing and Reading using {\small rs}EGFPs} % 3 steps smaller
\end{document}

\smallaffects the size of the type, not whether the subsequent word is set in lowercase (or uppercase). – Mico Aug 6 '12 at 23:40