They're using Bootstrap 3 grid sizing to their container div. You can override that CSS with your own changing their width or you'll need to make your own CB template and adjust it there. It is possible to edit the default CB template to change this, but your changes will just be lost every time you update CB. The below might work to increase the column size by 1 for the dropdown and button.
Code:
.cbUserListHead .cbColumn9 {
width: 66.66666667%;
}
.cbUserListHead .cbColumn3 {
width: 33.33333333%;
}