16GB I believe, but there is no point going over 4GB.
The microSD card is for game and channel storage only. You will not be able to read files, such as videos and photos, from it. If you are interested in personal media playback, you can do that via USB on the Roku 2 XS model.
If you remove a microSD card from your Roku 2 in the future, any channels or games that were stored on it will have to be re-installed onto internal storage the next time you launch them.