You can for example just add a "col-xs-12" class to the <ul>
which holds the list items:
<div class="col-md-6" data-toggle="dropdown">
First column
<ul class="dropdown-menu col-xs-12" role="menu" aria-labelledby="dLabel">
<li>Insert your menus here</li>
<li>Insert your menus here</li>
<li>Insert your menus here</li>
<li>Insert your menus here</li>
<li>Insert your menus here</li>
<li>Insert your menus here</li>
</ul>
</div>
This worked ok on any screen resolution in my site. The class will match the list width to it's containing div I believe: