Regallo glider seems to be in my head a lot, but I can't seem to find a bit of software that would let me simulate one.
Basically, the rocket goes up so high, tips over somehow, and comes back down. As it's almost all the way up, we hope a recovery device gets deployed. So why not a deployable delta wing and set of skids? Or wheels? I've seen big kites, they're often called sails. They're so good, they're still used since the stone age. Now that we have space aged materials, they can be used to our advantage, and out pops a wing with only a leading edge & root supported. The same could be done with a good sized rocket. All the pieces come back to base. With GPS, they could fly themselves home. A string of LED's could show them the way to the runway. Line follower robots have been around since the '70s. Why not put wings on one? Delta is a really forgiving configuration, it just has to land nose up. Not a problem for a robot.
Smallest I can think of would be big Estes tubing. Isn't that something like 2.6"? That might do.
Freewriting here about rockets, I might come up with something fun!