AFAIK you can set the width within the column definitions, for
example:
{display: 'Order Date', name : 'Ord_OrderDate', width : 60, sortable :
true, align: 'left', process: openSendBox},
Now I'm not sure if you can set width to 100% for instance. You could
try though... maybe push in there the word "auto" and see what
happens.
Remember, 90% of the grid is in stylesheets