Show correct size of edit form for HTML Block

Before:
![before](https://cloud.githubusercontent.com/assets/928116/2972547/75735b4e-db75-11e3-9a4f-87a65dfa2b84.png)

After:
![after](https://cloud.githubusercontent.com/assets/928116/2972555/7c287c08-db75-11e3-8cee-17b13d4bb700.png)