Hi Damien,
There is no MoTeC drawing showing wire sizes. As a guide we publish the wire ratings of M22759/16 for various wire guages in the appendix of our ECU and dash manuals. The insulation material is Tefzel and is rated to a maximum temperature of 150C.
In terms of appropriate wire sizes you're quite right there is nothing concrete, there are experts out there using 24 guage for sensors to lighten the weight of their looms and swear by this. 22 guage is also ok but again it comes down to the quality of wire and particularly insulation properties... expect to see more discussion on this from others on the forum...