By default, in Exchange and Outlook versions after 2003, everything is always done in Unicode mode, with unlimited maximum mailbox/PST/OST size, not counting higher-level server quotas and the like. However, after migrations, older settings may remain, and mailbox size maximums will often eventually crop up because users which existed before migrations may still be in non-Unicode mode, which incurs a hard max on their mailbox size.
The best solution this writer has found, is in Group Policy:
http://technet.microsoft.com/en-us/library/cc179012%28v=office.12%29.aspx#EnforceUnicode
One does have to install the Outlook ADM template into Group Policy.