JAVASCRIPT

set width of jqgrid

 $('#gridId').jqGrid('setGridWidth', '800');

set width of jqgrid

 var DataGrid = $('#gridId');

 //sets the grid size initially
 DataGrid.jqGrid('setGridWidth', parseInt($(window).width()) - 20);    

 //handles the grid resize on window resize
 $(window).resize(function () { 
       DataGrid.jqGrid('setGridWidth', parseInt($(window).width()) - 20); 
 });