You are right that the available page sizes is restricted. This is reflected
in VBA by the fact that there is a fixed set of page size constants, and
unfortunately, as usual in this kind of scenario, no facility to let you
define anything else.

What I think you will have to do is create a document with the correct paper
size, then create your own table layout with one cell for each label,
adjusting the various cell properties as required in Table|Table Properties
etc. Then put a { NEXT } field in every cell except the first. Then proceed
as usual. A test here suggests that that will work.

Instead, you could try starting with a custom layout on a "supported" paper
size, then change the paper size and create new columns and rows. However,
what I found when I did this was that the proagate labels function put
copies of the first label in every table cell, not just the cells intended
to contain label data, which means the output will look as if every other
label has been skipped. However, it is possible that I did something wrong
when I did this and that there is a way to make it work as you might hope.

Peter Jamieson