// get information about all columns.
final TableColumnModel columnModel = jTable.getColumnModel();

// setting column widths:
columnModel.getColumn( col ).setPreferredWidth( widthInPixels );
columnModel.getColumn( col ).setMinWidth( widthInPixels );
columnModel.getColumn( col ).setMaxWidth( widthInPixels );

// adding a bit of extra space between the columns.
columnModel.setColumnMargin( 5 );