Unfortunately, that's not possible. The new Page Plugin has a max width of 500px. Any width you specify higher than that is ignored.
The new page module does work on responsive layouts though, so if the Page Plugin is loaded into a column with a smaller width than specified (like on a mobile phone), it will automatically down-size itself instead of overflowing.
For more details on the specifics of the Page Plugin parameters, see:
developers.facebook.com/docs/plugins/page-plugin
I hope that helps explain,
Alex