Do you mean for actual number of merge fields in a
document or actual number of characters to merge into a
merge field?  Doug is right if you are looking for just
characters from a "text" merge field from Access.

If you are looking for maximum possible number of
characters per merge field, in Access you can get up to
655 characters if you use a Memo field for your merge
field.

I send out merge letters with paragraphs of information
saved in Access.