代码：
String[] arr={"http://itts.ihe.ufo.com/rtts/ws.jsp","http://ittsstg.ihe.ufo.com/rtts/ws.jsp"};
urlCbo=new JComboBox(arr);
urlCbo.setEditable(true);
urlCbo.setFont(new Font("Arial",Font.PLAIN,16));
urlCbo.setSize(new Dimension(800, 40));
urlCbo.setPreferredSize(new Dimension(800, 30));// 这里就是设置JComboBox宽度的代码
效果示例：
