See this http://answers.splunk.com/answers/114078/splunk-6-dropdown-menu-decrease-width.html
Alternatively, you can use the !important tag to supercharge any default CSS settings (thanks to Johan!).
Put this directly in the .css referenced in your dashboard:
.select2-container{min-width:500px !important;margin-bottom:8px;}
For a multiselect dropdown:
.select2-container-multi{min-width:500px !important;margin-bottom:8px;}
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