Pop the site size in automatically once it's been calculated

Currently you have to reload the page.

![screen shot 2014-11-10 at 14 21 53](https://cloud.githubusercontent.com/assets/308507/4979391/bfbf6c6a-68f6-11e4-8311-1e9139b24328.png)

This worked on 2.6

Also handles https://github.com/humanmade/backupwordpress/issues/589