I have a jqGrid with a lot of columns, and it shows up like this when the sum of the column widths are bigger than the fixed width container:
you can get horizontal scroll bar with static or auto size fields by wrapping the grid and pager with a div and run a small script,
you can see the jsfiddle-example, or just go like so:
and run this script:
$('#grid_container div:not(.ui-jqgrid-titlebar)').width("100%");
my example is using the script from trirand.com but iv tried it on their newest release and it worked just fine.
hope i've helped.