I would expect a lava tube from a drained lava flow to vary greatly. We see many have collapsed over the ages. It would provide an insulated underground tube miles in potential extent. That said, the danger of a roof fall would be low in some spots and ready to fall anytime in others. This is still rock, about 6 tonnes earth mass per cubic meter (one tonne on the moon). That means a very strong tunnel liner is needed. It also needs to be air tight. Research into the use of regolith as an ingredient of a concrete needs to be done. One traditional concreted mass is baking bricks of regolith. More research into brick baking from various lunar materials is also needed. A blown up fabric habitat would work inside this sealed brick tunnel.