As far as I can see it's a Word limitation that prevents you from specifying
an envelope size smaller than 6.4in in the Envelopes and Labels dialog box.
Strange, because it lists E6 size which is 110mm x 155mm and whose width is
therefore under the minimum allowed in the custom size.
I can't see a way around that /in this dialog/.
What I'd probably do is
a. create an A6-sized document based loosely on the layout created by the
Envelopes and Labels function, with the return address already filled in if
all your users use the same one, or leave it blank.
b. save it as a Word template (.dot) if your users are used to working with
them, or as a .doc if that's what they know
c. create a new toolbar in the .dot/.doc you just saved, and drag the
Insert Address button to it