Why initialize byte array to 1024 while reading or writing a file?

In java input or output stream , there always has a byte array size of 1024. Just like below:

        URL url = new URL(src);
        URLConnection connection = url.openConnection();
        InputStream is = connection.getInputStream();
        OutputStream os = new FileOutputStream("D:\\images"+"\\"+getName(src)+getExtension(src)); 
        byte[] byteArray = new byte[1024];
        int len = 0; 
        while((len = is.read(byteArray))!= -1){
            os.write(byteArray, 0, len);
        }
        is.close();
        os.close();

Why initialize this array to 1024?