Advice on restricting SIZE of InetAddress in INDEX?

[Bill Fenner <fenner@research.att.com>]

The DESCRIPTION of InetAddress says:

            When this textual convention is used as the syntax of an
            index object, there may be issues with the limit of 128
            sub-identifiers specified in SMIv2, STD 58. In this case,
            the OBJECT-TYPE declaration MUST include a 'SIZE' clause
            to limit the number of potential instance sub-identifiers."

How do I include a SIZE and still retain the flexibility of supporting
future types?  Include the max SIZE that will fit (e.g. I have two
InetAddresses in an INDEX so make them SIZE(1..120))?  Or, since I
only expect to see IPv4 and IPv6 addresses, make them SIZE(4|16|20)?

Thanks,
  Bill