Adjust BlockPicker's Preview height
Currently the preview height is fixed to 110px and there is no way to change it.

![image](https://user-images.githubusercontent.com/45073703/112842293-39f03c80-90c3-11eb-9384-de97b3eb6233.png)

![image](https://user-images.githubusercontent.com/45073703/112842544-7a4fba80-90c3-11eb-843e-1d373cf1a985.png)

Adding a class to preview div would be great. Thus way it can be customized.
