I don't know of a specification sheet that has the wire size. In general the individual wires are one size larger (smaller gauge number) than 12 volt as the current is twice 6 volt.

@chip  I get that, Question So 18 gauge would go 16 gauge ? 12 would go to 10 ? Thank you for the reply.

In a word yes. See the website below for the wire size for 12 volts. Double the amperage and then select the wire size for 6 volts. If there is a question a larger (lower gauge) wire has a safety factor.

https://learnmetrics.com/12v-wire-size-calculator-chart/

 I generally use the following chart as a guide however after disassembling and measuring both a 1941 and 1948 harness, I find in many cases it’s the smaller gauge (larger number).  I have gauge, color, length, and original end configuration for each individual wire.

Nice chart I will use as my guide. Thank you

@chip  Thank you for the reply

Be careful what wire you are purchasing.  When I was wiring my car I kept seeing copper clad wiring (CCA) and copper wiring.  The copper clad wire was cheaper but the amperage rating was not as high.  I just went with copper stranded wire.  Also for anything in the engine bay make sure the shielding is heat and oil resistant.  

Have several colors available and document every color as you go.  We staples our documents to the back of our shop manual.  If you have to use the same color for multiple circuits mark it on the wire every few inches.  We used colored electrical tape to apply a loop every 6 inches.  Future you will thank you if there are any electrical issues.  I found this out the hard way.  I once worked on a car that had been rewired in all yellow wire, that was a nightmare to trace.

This is your choice but it saved me one morning.  I did not use the old style headlight switch with a fuse in my 41.  I used a universal replacement that had a breaker in it.  The reason for this was that if a fuse goes out while you are driving it will not come back on until you replace it.  With the breaker if it trips you can shut the lights off, wait for the click and turn on the minimum needed set of lights to hopefully get you home.  I had mine trip in the dark while going through a curve at 30 MPH.  My fault as I had the fogs and the headlights on the same circuit.  Shut them all off, heard the click and turned on only the headlights.  If it was a fuse that momentary bit of darkness would have been longer.  

For battery cables do not use the kits sold in Auto Zone etc.  These are to small. 

I purchased an Inova circuit tester when wiring my car.  This unit can work on 5v or 12v.  Great tool for testing out new wiring as you go.  

@33guy    https://chevy.oldcarmanualproject.com/chevywiring/pdf/48car.pdf

This is a diagram of my '48 that shows' the gauge of wire used in each circuit, might help.