[Plugin: WordPress Form Manager] How to add scrollbar to list's dropdown option

Hi,
First I would like to commend the developer of this plugin as it is a vital part of my development arsenal. I am attempting to create a dropdown displaying abbreviations of all the US states using the list form element. The problem is that when all of the states are loaded there is no scrollbar, and instead all 50 are displayed making for quite an extremely long dropdown. How can a scrollbar be added to reduce how many are displayed at any given time? Thank you for your attention to this by the way.

http://wordpress.org/extend/plugins/wordpress-form-manager/