Update menu button's height and width to 48px to match the header bar's height

I guess I got the height wrong, or maybe it changed after I wrote the settings menu pull request.

Now:

![screenshot from 2013-11-02 19 35 38](https://f.cloud.github.com/assets/1447206/1460244/15d356d6-4420-11e3-9d19-ee173a645f32.png)

Fixed:

![screenshot from 2013-11-02 19 36 49](https://f.cloud.github.com/assets/1447206/1460245/198a3c4a-4420-11e3-9828-e8065728a1a0.png)