I use
\bibliographystyle{abbrv}
{\small \bibliography{reference}}
for references. How can I change the size of the "Reference" letter?

I use
\bibliographystyle{abbrv}
{\small \bibliography{reference}}
for references. How can I change the size of the "Reference" letter?

The bibliography uses either \chapter* or \section*, depending on the class you use. You can rewrite them as explained here. However, if you are using any package such as natbib or biblatex, you can explicitly define it. Say you are using article and want to have it displayed as a subsection:
\renewcommand{\bibsection}{\subsection*{\bibname}} %natbib
\defbibheading{bibliography}[\bibname]{\subsection*{#1}} %biblatex
Since the better approach depends on the packages you are actually using, as @Aradnix commented, it would be easier to get waht you need if you provide a MWE.
\documentclass{...}and ending with\end{document}. You might also want to add to your question how you're including the pdfs etc. Why do you use thebiblatextag? – Aradnix Sep 28 '14 at 20:22