What is the proper way to size neutral wire

So you have 20A 120V to share in a panel. Lets say you put a 1A fuse on the hot in there to protect a component and enable you to use thinner gauge wire. What allows you to use thinner gauge wire on the neutral?

My understanding is that the neutral will be the combined load of all the active components in the panel. That would make me think that your neutral must be sized accordingly.

Is this a dumb question?

Well you'd have two options. You could just use 12 ga for all neutrals. But if the goal of your fuse is specifically to let you use a thinner gauge wire downstream, then I supposed you could also use the same gauge for the neutral return for whatever is powered off the 1 amp fuse. But not for anything else. Like if you have a hot bus and neutral bus and you're going to have a pump or something else running the pump's neutral should go right to the bus.

If I ever build such a panel, I would not use anything smaller than 14 ga. Some industrial companies don't allow anything less than 14 ga. to be used in their panels because it isn't considered to be durable enough for industrial applications. Plus of course they are far more concerned about the hundreds or thousands of dollars an hour of downtime to a panel moreso than an extra $20 for using a heavier gauge wire in it.