def block_sizes(max_size):
    """
    Return all possible block sizes of an inter-predicted frame (min size is 4x8 or 8x4)
    :param max_size: maximum exponent of the power of 2 specifying width/height (e.g. 6 = 32x32), max value is 8
    :return: string list of "width x height" sizes
    """
    if max_size > 8:
        raise ValueError("Invalid max_size value specified!")
    else:
        return [f"{2**x}x{2**y}" for x in range(2, max_size) for y in range(2, max_size) if x != 2 or y != 2]