for root, _, files in os.walk('/tmp/datasets/training/negative'):
    for f in files:
        fullpath = os.path.join(root, f)
        try:
            if os.path.getsize(fullpath) == 4 * 1024:   #set file size in kb
                print (fullpath)
                os.remove(fullpath)
        except WindowsError:
            print ("Error")