How to increase jqgrid header checkbox size

Free jqgrid row height and font size is increased using styles from answers from JQgrid set row height and How to make free jqrid font awesome action buttons bigger

/* http://ift.tt/1ESnAn3
*/
.ui-jqgrid .ui-jqgrid-htable th {
    height: 2em;
    font-size: 1.2em;
}

.ui-jqgrid tr.jqgrow td {
    height: 2.8em;
}


/*http://ift.tt/1PW1plx
*/
.jqgrow .ui-jqgrid-actions > .ui-pg-div > span {
    font-size: 22px;
}

FontAwesome icon set and checkbox are used. Row selection column checkbox in grid column header is still too small:

How to make row selection column header checkbox to have same size as in rows ?