Are you asking about the size of the references list or the citations?
For a citation, you could enter it like this {\tiny\cite{xyz}}. For
the references list, the trick will depend on which bibliographystyle
you are using. I am using APA format, and my apacite.sty redefines
some of the mechanisms in beamer, so I used an ugly hack.
I edited the \thebibligraphy macro in apacite.sty and added a \tiny
that was wrapped in an \if test so that the \tiny would expand only if
I use the beamer package. Here the part of the \thebibligraphy macro
that generates the \list
(lines from the \thebibliography macro omitted...)
\list{\relax}{\labelsep=0em%
% \parskip\z@ \@plus .3\p@\relax
\parsep=\bibparsep%
\ifx\***@version\@undefined% ******* I added this
if-test for beamer package
\else
\tiny%********* This command will run only if the
document is using beamer
\fi
(more lines from the \the bibliography are omitted)
**If you did not override the bibliography style**, and if you are
using the beamer document class, you might be able to use the macros
that are unique to beamer.
it might looks something like this:
(this par is in the preamble)
\setbeamerfont{bibliography entry author}{size=\tiny}% possible add
more formatting including italics where needed
\setbeamerfont{bibliography entry location}{size=\tiny}
\setbeamerfont{bibliography entry note}{size=\tiny}
(etc.)
(this part is at the end of the document where you wan the
biblography...)
\bibliographystyle{WHATEVER-STYLE-YOU-ARE-USING}% I use apacite, which
requires apacite.sty
\usebeamertemplate{bibliography entry author}%
\usebeamertemplate{bibliography entry title}%
\usebeamertemplate{bibliography entry location}%
\usebeamertemplate{bibliography entry note}%
\bibliography{MyBibDatafile}