Evaluating old Wiring Harness

I will be replacing it with new, but wondered if i needed to increase the gauge of the high amp wires. Sounds like not if i go new. I am curious about the new harnesses as i know there are differences between the various models and options. I dont need any AC wiring and have manual trans so the reverse light switch is wired with a separate harness.... some of the harnesses dont ask enough questions, so wonder if its just one new harness with everything (one size fits all) and not truely one that replicates stock (several options, sub harnesses, and such).