There has been a recent conversation on this topic - see
http://groups.google.co.uk/group/microsoft.public.word.mailmerge.fields/browse_f
rm/thread/f877ab6c115da8e3/894cb8508515a02a?lnk=st&q=&rnum=1#894cb8508515a02a

(You may need to reconstitute the link)

Which connection method are you using to connect to Access (if you don't
know, it's probably OLE DB)? Can you try the other methods (check Word
Tools|Options|General|"Confirm conversions at open", go through the process
of connecting to your data source again, and choose DDE, ODBC or OLEDB)?

Are you connecting to a Query or a Table?

Experiments here with up-to-date versions of Word 2003, Access 2003, and an
Access 2000 format database suggest that all connection methods (the
default, OLE DB, ODBC, and the old Word 2000 default, DDE) read memos
considerably longer than 255 characters, but that DDE fails altogether with
records with long memo fields. But it is obvious that other people are
experiencing something different.

Peter Jamieson