what size fuel line for nitrous setup?

When it comes to fuel, it's better to have too much reserve than not enough.

A 140gph pump and 1/2" or -8 line works well for most any street car. If you start pounding juice, then you may need to rethink the pump size, a 1/2" or -8 line will provide enough fuel for some pretty serious HP.

You could even go -8 from cell to pump , then 3/8 or -6 to regulator and carb. I wouldn't do it, but, it's an option.

My attitude, if you are going to do the lines, put 1/2" in so you don't potentially have to do the work twice.