What should I do now that my tomatoes have outgrown the netting?

I’d keep the sides protected and either remove the roof or raise it with a simple frame extension. A short cage can press down on the stems and make it awkward to get inside, and a dense cover can limit airflow. If you extend it, leave enough space to reach the fruit and check the plants.

Give the stems some support as they get heavier, too. Determinate plants can still outgrow a low enclosure, and semi-determinate ones may keep growing for longer.

Determinate doesn’t necessarily mean compact. Varieties can get taller and wider than their label makes you expect, and “semi-determinate” is a clue that the San Marzano may keep stretching beyond a neat bush shape. I’d check the variety description if you still have it, then plan the cage around the plants you actually have rather than the category alone.

I wouldn’t aggressively prune just to make everything fit. You can redirect or loosely tie stems to a taller support, but cutting back growth may also mean cutting off future flowers or fruit.

If the netting is mainly there for fruit thieves, you could leave the sides up and try the top open, especially if you don’t have larger animals reaching in. Keep an eye on what’s actually eating the tomatoes. If the damage is mostly to ripe fruit, picking them as they start to color and letting them finish indoors may help, though it won’t prevent every kind of pest damage.