Create a document with an envelope as that size and save it as a template.
Then use a formletter type mailmerge based on a document created from that
template. The only thing different about an Envelope type mailmerge is that
it gives you the opportunity to specify which envelope you want to use.

Signature
Hope this helps.
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services on a paid consulting basis.
Doug Robbins - Word MVP
> We use A6 (6.5" x 4.75") envelopes frequently, and this is a pretty
> standard
> size. Is there a way to download or create addtional envelope entries in
> the
> envelope options, or do we have to do a custom setting for each merge?