CABLE SIZING

For maximum cable size for a 4-20mA signal, I wouldn't use much bigger than a 2AWG.... because anything bigger would be really silly. As for what I suspect you are wanting is Minimum size... I'm not really sure. I usually just use 16-18AWG for my 4-20mA signals because it is the most convenient size. As for the rest, it sounds like you are asking homework questions, and I don't know the answers off the top of my head, so do what I would do and ask Google.

A0) Please turn off caps lock. Capitals use IS CONSIDERED SHOUTING AND RUDE.

A1) 0.75mm² is a typical size used in industry in the UK. It is based upon the mechanical robustness of the conductors more than anything else. Any bigger and it won't go in the terminals on many commercially-available instruments.

A2) For a 4-20mA current loop, it really doesn't matter much unless the cable length starts getting up into the >1km region, after which it needs to be considered though the effect will sometimes be small; 20mA into 100Ω is only 2V, after all. There are a number of techniques used to overcome excessive loop resistance, including current repeaters, fibre-optic transmission and voltage sensing and boosting circuits.

A3) Almost linear.

The total resistance of the loop shall be less than 1200 Ohms, including the impedance of the generator...