Basically there are 2 way to do the change:
1) When you create the grid, use the property width with value in pixel as width: width you want>
2) After grid creation, use setGridWidth(width, shrink) API as
$([tableid]).setGridWidth(<Width you want>,true);
Similarly, height of the table can also be set using height :<value is pixel> and setGridHeight(height) API.