Just given the case that somebody had the same problem...
That's what I was looking for (at least the code does what i wanted):

myTable = new JTable() {
public Dimension getPreferredSize() {
    int HeightSum=0;
        for(int i=0; i<getRowCount(); i++){
            HeightSum+=getRowHeight(i);
        }
        return new Dimension(SizeOfSolumns(),HeightSum);
}
public Dimension getPreferredScrollableViewportSize(){
    return getPreferredSize();
}

public int SizeOfColumns() {
    int WidthSum=0;
    for (int i=0; i<getColumnCount(); i++) {
        WidthSum+=SizeOfColumn(i);
    }
    if(getParent().getSize().width<WidthSum)
        return WidthSum;
    return getParent().getSize().width;
}
public int SizeOfColumn(int vColIndex) {
    DefaultTableColumnModel colModel =
        (DefaultTableColumnModel)getColumnModel();
    TableColumn col = colModel.getColumn(vColIndex);
    int width = 0;

    TableCellRenderer renderer = col.getHeaderRenderer();
    if (renderer == null) {
        renderer = getTableHeader().getDefaultRenderer();
    }
    Component comp = renderer.getTableCellRendererComponent(
        this, col.getHeaderValue(), false, false, 0, 0);
    width=comp.getSize().width;

    return width;
  }
};

Regards,

Maximilian