1. It's ok to eat some "junk food" as part of your diet. That's not going to automatically kill your diet, if you still control calories. It might not be as satiating as eating other foods though, so it's worth experimenting with this. (personally I would still eat "junk food" on my diet and I enjoyed the extra flexibility).
2. Just cause you're eating junk food, since calories is what ultimately matters, it's still worth not ordering the large. Saying " well it's junk food I'm already lost" and then eating a larger portion is just making the sustain much worse than it needs to be.
3. Similarly, just cause it's a junk food place, doesn't mean there aren't better or worse options. I love McDonald's, and still eat there even when dieting to lose weight. But I'll eat only chicken nuggets, and skip the fries. Not nearly as good or healthy as eating other sources of protein, but definitely better than including an extra 500 calories of non satiating carbs.
I think if 'order the large' is an option, you're already ordering from a takeaway/in a 'junk food' place.