Nor all size accepted in size() method (like fa-xs, fa-10x)

[Molnár Sándor]

There is fa-xs, fa-sm, fa-7x, fa-10x but size() method throw error:
FontAwesome::size() - invalid value. Use one of the constants: FontAwesome::SIZE_LARGE, FontAwesome::SIZE_2X, FontAwesome::SIZE_3X, FontAwesome::SIZE_4X, FontAwesome::SIZE_5X.

[Revin Roman]

Sizes added in 3.3.0 version. Thanks for issue.