Resize default catagory widget dropdown

I am using the default category widget and when the option to show it as a drop down is ticked, the dropdown spreads across 2 columns.

Can someone direct me to the line of code where the size of this widget can be corrected.

An example of what I am referring to is here at http://mexicotrucker.com

Also, I would be interested if anyone knows of a plugin that would control the cats with ajax or something similar.