Change Search Forums Formatting

I have just added a Search Forums widget to my Forums pages. I have a lot of content in my sidebars and would like to reduce the height of this widget. There is just too much wasted space.

Please see attached screenshots of how it is now, and then how I would like it to be.

I have tried to adjust using Custom CSS but with no success. Can you tell me please how I can accomplish this.

Thanks!