How do you decide the correct trace width in PCB design?

There is not a single universal rule for trace width. It really depends on what the trace is doing. In practice, these are the main things you can look at:

1. Current: For power traces, current is usually the biggest factor. You want to avoid excessive heating and voltage drop. The usual approach is to use a trace-width calculator.

2. Impedance (for high-speed signals): For USB, HDMI, RF, etc., trace width isn’t chosen for current — it’s chosen to hit a target impedance. This depends on the PCB stack-up, so you calculate (or let your CAD tool do it).

3. Layer & copper thickness: Inner layers need wider traces for the same current, since they can't dissipate heat as effectively.

4. Manufacturer capabilities: Your fab sets the minimum trace/space. Always check their design rules.

5. Noise / sensitive signals: Analog traces, reference lines, sensor signals, etc., sometimes benefit from slightly wider traces to reduce resistance/noise pickup. Very thin long traces can behave like antennas.

6. Mechanical reliability: Traces going to connectors, test pads, or anything that might experience mechanical stress are often made wider to reduce the risk of cracking or pad lift.

And of course, in electronics there are always other considerations that might not be obvious at first.

Use the largest trace or pour that fits the board size and clearance rules, unless:

• impedance control forces you to use a specific trace width
• solderability would be compromised too much (usually not a concern in a reflow oven or hotplate)

As Rick Hartley put it once: "You've paid for all that copper, so remove as little as necessary!"

You might find your answers from a suitable PCB design software tool. You can then enter currents, frequencies etc. and get PCB specifications for tracks, vias and all sorts.

Try the free Saturn PCB Design Toolkit, looks like this.