#wpbody #wpbody-content #dashboard-widgets .postbox-container {
  width:100%;
}
@media ( min-width:768px ) {
  #wpbody #wpbody-content #dashboard-widgets:not(.columns-1) .postbox-container {
    width:50%;
  }
}
@media ( min-width:1200px ) {
  .columns-prefs .columns-prefs-3,
  .columns-prefs .columns-prefs-4 {
    display:inline-block;
  }
  #wpbody #wpbody-content #dashboard-widgets.columns-3 .postbox-container {
    width:33.3%;
  }
  #wpbody #wpbody-content #dashboard-widgets.columns-4 .postbox-container {
    width:25%;
  }
}