Yes Joe, you are right Active Directory does not check the maximum size for this attribute, this is restriction of ADUC UI.
if someone is interested, format for sip_uri can be described by regexp like this:
^[sS][iI][pP]:(\w{0,194})@(((\w{1,63}\.){1,3}[A-Za-z]\w{0,62}$)|(\w{1,63}))$ I calculate it by experimentation.
total length is 454.
This also corresponds to the length of attribute msRTCSIP-PrimaryUserAddress. here is fragment from it schema:
dn: CN=ms-RTC-SIP-PrimaryUserAddress,CN=Schema,CN=Configuration,DC=X
changetype: add
adminDescription: msRTCSIP-PrimaryUserAddress
adminDisplayName: msRTCSIP-PrimaryUserAddress
description: Valid SIP URI.
objectclass: attributeSchema
attributeID: 1.2.840.113556.1.6.24.1.1
#schemaIDGUID:{45FC6F43-C8EB-40d4-91F3-763C46F6F250}
schemaIDGUID:: RfxvQ8jrQNSR83Y8RvbyUA==
oMSyntax: 64
attributeSyntax: 2.5.5.12
rangeLower: 0
rangeUpper: 454
isSingleValued: TRUE
searchFlags: 5
isMemberOfPartialAttributeSet: TRUE
ldapDisplayName: msRTCSIP-PrimaryUserAddress
#Base 64 Encoded GUID of :E2D6986B2C7F4CDA9851D5B5F3FB6706
attributeSecurityGUID:: a5jW4n8s2kyYUdW18/tnBg==
i believe that ms-RTC-SIP-PrimaryUserAddress has the same syntax.