你应该像这样写简单的代码

//create your DatePicker
       DatePicker picker=new DatePicker(new Date(), new SimpleDateFormat("dd/M/yyyy"));
       //to change component size 
       picker.setPreferredSize(width,height)//it will work depends your layout
       //to change font and text size on picker JTextField
       picker.getComponent(0).setFont(yourFont);

希望对你有帮助！