def format_size(size):
    """
    Formats a size in bytes.

    >>> format_size(123)
    '123 B'
    >>> format_size(123456)
    '123 kB'
    >>> format_size(123456789)
    '123 MB'
    >>> format_size(123456789012)
    '123 GB'
    >>> format_size(123456789012345)
    '123 TB'
    """
    prefixes = ['k', 'M', 'G', 'T']
    for pos, prefix in reversed(list(enumerate(prefixes))):
        base = 10 ** ((pos + 1) * 3)
        if size >= base:
            return '%i %sB' % (size / base, prefix)
    return '%i B' % size