For length, see the optional rangeUpper attribute that is associated with the ms-Exch-Extension-Attribute-1 attribute.
For Unicode strings, rangeUpper specifies the maximum number of characters. In the case of the ms-Exch-Extension-Attribute-1 attribute, it is 1024 characters.
The type of the attribute is determined by the combination of the attributeSyntax (2.5.5.12) and the omSyntax (64). In this case, a unicode string.
The attribute is replicated to the global catalog (isMemberOfPartialAttributeSet=true)
[MS-ADTS]: Active Directory Technical Specification: 3.1.1.2.2.2 LDAP Representations
http://msdn.microsoft.com/en-us/library/cc223177%28v=prot.13%29.aspx
Attributes Added by Exchange
http://msdn.microsoft.com/en-us/library/ms869146%28v=exchg.65%29.aspx
ms-Exch-Extension-Attribute-1 Attribute
http://msdn.microsoft.com/en-us/library/ms980473%28v=exchg.65%29.aspx