If you want to apply reasonably good engineering practices here’s some suggestions. I size wires based on 1) the temperature rise due to current flowing through the wire 2) voltage drop (voltage drop = ohms/foot for your wire * current in the wire * round trip wire length 3) power/energy loss (power loss = current in the wire ^2 * ohms/foot * round trip wire length, energy loss is power loss * number of seconds you are actually running that current through the wire). You need to size the wire so the temperature rise is less than what the wire insulation is rated for. Teflon can go up to 100 degC or more, PVC is something like 85 degC. You need to check the spec on the wire you’re using. I don’t want anything that hot running around my system so I keep the temperature rise much lower. What an acceptable voltage drop is depends on a lot of things. Signals may meet the minimum voltage level at the TX side of the cable and drop below the minimum acceptable voltage on the RX side if there is too much voltage drop. Linear regulators, DC/DC converters, etc. also have minimum acceptable input voltages. Power/energy loss matters on a battery powered system. In most of my cases, this is the overriding concern. I don’t want to waste a lot of my battery resources heating up wires so I try to keep my power/energy losses less than 1% of the total energy in the battery pack.
Signal integrity is a whole separate and complex subject. One of the best thing you can do is use shielded twisted pairs. Shielding is expensive and not always necessary but twisted pairs can provide a controlled impedance path which is important for signal integrity. USB compliant cables are twisted to maintain roughly 100 ohm impedance. And you can get cables with multiple shielded and/or unshielded pairs of the same or different wire size.
Ribbon cables are the worst possible choice for signal integrity but obviously work in a lot of situations. The unfortunate thing about the signal integrity issue is if you just hack a system together, it may work in your factory but not in another, more noisy, factory and now you have unhappy customers.
Seems like a lot of stuff to worry about, right?. The good news is none of the above is hard to do and once you do it the first time, it becomes pretty routine.