Hold on a minute here.....
If I remember correctly the idea is a 2 piece wing with a tube joiner. In that case ..
Yes, the wing tube is 3/4"
NO, you don't buy a 3/4" tube.
Remember, for this to work you are not dealing with a tube, but 2. One inside the other. To end up with a 3/4" diameter tube to match your spar, you need an outer sleeve that is 3/4" outside diameter, For that to work you will be buying a tube with about an outside diameter of 5/8" or so. You will wrap it in fiberglass and epoxy and make a sleeve that is glued into the wing. I would probably use 3 layers of about 8 ounce fabric. This will give you a sleeve that is a tiny bit shy of the 3/4" mark. This will be fine. If you refer back to Bruces illustration (He is just showing off by posting those drawings by the way) you will notice that if the sleeve is a bit smaller that the width of the spar the gap will be filled with the resin. If the sleeve is even a tiny bit too large, when you glue the 1/16 plywood to the spars it will put enough pressure on the sleeve to flex it making it just about impossible to insert the tube.....
So you need to go looking for a 5/8" ish OD tube. I use either carbon or titanium, so hopefully Bruce of Shawn will chime in with an answer to your question of wall thickness on the tube.
Scott