Steam Engine Cylinders

[Steamtrains]

I've been browsing this site (<!-- m --><a class="postlink" href="http://www.railarchive.net/randomsteam/dmir1311.htm">http://www.railarchive.net/randomsteam/dmir1311.htm</a><!-- m -->) looking to get some ideas for upgrading my steam roster, and I see that cylinder dimensions are given as "27"x 32", or some such numbers. What do these numbers refer to..??

Thanks for any insights....