@Html.ListBoxFor(vm => vm.SelectedUsers, Model.UserSelectList, "--- All ---")

.listBox
{
   width: 200px; //set your width or make it auto
}

@Html.ListBoxFor(vm => vm.SelectedUsers, Model.UserSelectList,new { @class = "listBox" });

Html.ListBoxFor(vm => vm.SelectedUsers, Model.UserSelectList, new { data-placeholder="-- All --", style = "width: 100%"});