Depending on what you are planning to do.
Plug-in wings (tube going through the fuse), plug-in one piece wing. etc.
Balsa built up wing? or Foam cores?

If I put carbon fiber tube on 40 size, I would go with 5/8" or 1/2".

Your dimensions are very similar to EZ ARFs.
It is still better to make the wing one piece for the small size plane.

Your Idea counts here!!
I often wanted a small plane 40 size that I can disassemble stab, wing, fuse in halves and go in a small rectangular case. I know there is a 2M version Pattern plane available this way.

Good luck.