Generator B Harness Wire Size

I don't really like the idea of an overvoltage cut-out between the Stator and Regulator, to me the additional failure point outweighs any potential benefit, but that's just my opinion.  Regarding your question...

Service Bulletin SI-915i-006 / SI-912i-024 introduced a new larger connector for the 915 regulator B, which is also optional for the 912iS regulator B connector. This connector is shipped with pins that are rated for wires from 12 AWG to 10 AWG.  I installed one of these kits on a 912iS and the pins seemed to fit the wires correctly. If I remember right, the wires on the regulator side of the connector were smaller than the wires on the stator side. It may have been #12 on the stator side and #14 on the regulator side, but the pins shipped with the kit crimped down tight on both. I used the  Rotax supplied kit, and the connectors shipped had a Rotax part number on the bag, but I recognized them as Amphenol.  Specifically (and unofficially), the connectors are:

Male Plug:  Amphenol RT06164PNHEC

Female Connector: Amphenol RT07164PNHEC

Male Pin: Amphenol MP10A23S (12-10 AWG)

Female Pin: Amphenol MS10A23S (12-10 AWG)

Note: The 12-10 AWG Pins are the only size available for this connector since it is rated for 45 Amp max current. 

Lastly if you do decide to go with this new higher amp connector, know that the connector pins shipped with the Rotax kit have an external stop collar that is just a few thousands of an inch too big for the crimper that Rotax recommends in the service letter (at least mine did not fit).  You can easily file a bit off of the circumference of the stop collar on the pins without compromising them, but it would be better to have a crimper with the larger diameter opening.  The Rotax Bulletin calls for a DMC AF8 tool with a UH2-5 positioner, however, the 5.6mm orifice in the tool won't quite accept these pins. The DMC MH860 would fit the pins correctly but it's an expensive tool. Also, JRDTools has an AF8 style crimper that is available with slightly larger diameter opening (6.35mm) that would work well for this. That part is a JRD-FT8, still expensive, but not as bad, and a great tool to have. 

The factory has historically been reluctant to release design info.
I am not condoning your decision to complicate the electrical system of the engine and would advise that well enough should be left alone.
But this is Experimental aviation and we love to reverse-engineer everything. 

The B gen is rated at 420 watts at 14.5 volts.
That is just a hair under 29 A DC.
The Stator is 3 phase current so dividing 29 amps by the square root of 3 yields just under 17 amps AC per phase.
16 awg wire is rated at 18 A at 90°C according to Wikipedia.
90°C is some mighty hot wire especially if installed in an engine compartment.
14 awg wire is rated at  20 A at 75°C; So both wires would be candidates but as you are extending the cable length the larger cable would be the better choice.

Take note that SB-912i-006 describes increasing the stator wire size but does not specifically state the actual size used.
I suspect they upgraded the 16awg equivalent from the previous 912ULS design to a 14awg metric equivalent.
So again the larger cable would be the better choice if you intend to perform this modification.
The choice is yours.
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The G label on the wires refers to the German word for Yellow. 
Yellow = Gelb, hence "G".  Three Gelb-Colored wires.

As one more data point, service instruction SI-915i-006 / SI-912i-024 instructs the installer of the new connector to set both the strip tool and the crimp tool to 12 AWG for both the Stator and Regulator side of the connector. 

https://legacy.rotaxowner.com/si_tb_info/serviceinfo/si-912i-024.pdf

Thanks very much for the replies, gents.  Lots to digest in your posts and the linked documents.  I'll have to read through it all and have a good think about it...