Specify attribute's minimum length

I'm implementing a new infrastructure which will include a kind of Microsoft's GPO using ldap, dns, dhcp and pam.

I'm creating a new schema to store the information I need, and I have a groupPolicyId attributeType which describe a 32 characters identifiers.

For this I did the following:

attributetype ( 1.3.6.1.4.1.39259.2.1.1 NAME 'groupPoliciId'
    DESC 'Group Policy Identifier'
    EQUALITY caseIgnoreMatch
    SUBSTR caseIgnoreSubstringsMatch
    SYNTAX 1.3.6.1.4.1.1466.115.121.1.15{32}
    )

Using the SYNTAX 1.3.6.1.4.1.1466.115.121.1.15{32}, I specify that I want an UTF-8 encoded string which is at most 32 characters.

I now need to set a minimum length, googled for hours and didn't find anything which may helps. Any Idea? Thanks.