Docs for IS_LENGTH

    Checks if length of field's value fits between given boundaries. Works
    for both text and file inputs.

    Arguments:

    maxsize: maximum allowed length / size
    minsize: minimum allowed length / size

    Examples::

        #Check if text string is shorter than 33 characters:
        INPUT(_type='text', _name='name', requires=IS_LENGTH(32))

        #Check if password string is longer than 5 characters:
        INPUT(_type='password', _name='name', requires=IS_LENGTH(minsize=6))

        #Check if uploaded file has size between 1KB and 1MB:
        INPUT(_type='file', _name='name', requires=IS_LENGTH(1048576, 1024))

        >>> IS_LENGTH()('')
        ('', None)
        >>> IS_LENGTH()('1234567890')
        ('1234567890', None)
        >>> IS_LENGTH(maxsize=5, minsize=0)('1234567890')  # too long
        ('1234567890', 'enter from 0 to 5 characters')
        >>> IS_LENGTH(maxsize=50, minsize=20)('1234567890')  # too short
        ('1234567890', 'enter from 20 to 50 characters')