would any one know what size in mm  wire that runs from plug  to electric brakes 

thank you

The 2 power wires which have a color code blue should be rated at 25 amps I hope that may be of some assistance to you

4mm  15amp is sufficient Terrola.

https://www.caravansplus.com.au/car-accessories/trailer-plugs-sockets/

HI
Use 4.5mmsq = is called 6mm ""auto cable"" [diameter including insulation] up to 10amps with moderate volt drop . This is the cable to use

1.84mmsq = ""auto cable"" 4mm. This is way to small

Teknosha use/recommend 10-12awg in different spots , 10awg =6mmsq and 12awg = 4mmsq