maxWidth

The maximum width which the column can take when user resize it. In order to use this property the resizing of the grid needs to be turned on.

$("#grid").shieldGrid({
    dataSource: {
        data: [
            { ID: 1, firstName: "name1", middleName: "middleName1", lastName: "lastName1" },
            { ID: 2, firstName: "name2", middleName: "middleName2", lastName: "lastName2"}
        ]
    },
    columns: [
        { field: "ID" },
        { field: "firstName" },
        { field: "middleName", resizable : false },
        { field: "lastName", minWidth: 100, maxWidth : 300 },
    ],
    scrolling: true,
    resizing: true
});

Live demo of column resizing you can find here: Column Resizing