I have created a workbook (A) that pulls data from workbooks (B) and
(C). If I combine A with B or C or both, the resulting workbook
becomes too large to email. If I email the files separately to other
users (who are not especially Excel literate), the links in A fail to
update, since they are pointing to files on my computer, not theirs.
I'm very reluctant to ask users to edit the links in A to point to the
correct files; I've got little confidence that will be successful.

Is there a way to specific relative, not absolute file path names, so
that as long as B and C are in the same directory as A, a remote user
can download all three files into one directory and have the links in
A work correctly?

I suspect that's how it works already.

As a test, there were two workbooks in folder f1. Book1 has links to Book2.
I sent two workbooks as separate attachments in an email message to myself.

After saving the two received attachments in folder f3, I could open Book1
and the links were updated to point to ...f3\[Book2.xls]... automatically.

I'm not sure exactly why or how this works. Maybe Excel keeps track of how
a link originally was defined in order to be helpful in this way. Granted,
it might work differently going from one computer to another.

(I have Excel 2003.)