Depends on which language you'd like: http://en.wikipedia.org/wiki/Wikipedia:Database_download

8 GB bzip2-compressed, for the English Wikipedia, if you download the dump that contains only the current revisions of article-space pages. Much bigger if you want the one with every historical revision.

Thanks. Important to mention that is text only.

small enough to have offline on your phone, as several apps do.

The text of a large percentage of the current version of English Wikipedia, without discussion pages, is that small.