How to change font size for bibliography?

Using the biblatex package you could also write

\renewcommand*{\bibfont}{\footnotesize}

(Personally I'd use \small instead of \footnotesize.)

First I have to say that you should avoid fiddling with font sizes in order to squish more text into a document. Conferences and journals often discourage such formatting hacks; some might even reject papers found to be doing this.

Anyway, if you still need to to this for whatever reason, the easiest way I found is to write something like

{\footnotesize
\bibliography{bibfile}}

And if you use the natbib package:

\def\bibfont{\footnotesize}