I know thicker/lower gauge is better for power transfer, and length for vehicle position flexibility...but at what point do you see diminishing gains? For instance, Amazon has:

20 foot, 4-gauge cables for $50
25 foot, 2-gauge cables for $100

My experiences to date have been with small cars, which have managed with lower grade thin jumper cables. No idea about what passes for "standard issue" when dealing with trucks. Thoughts/experience? Thanks in advance!

 

12 to 15 foot 4 gauge
20 to 25 foot 2 gauge

Keep in mind that 25 foot 2 gauge cables are pretty big/bulky. Going shorter is much less bulky because of both less length and gauge.

The Rover V8 isn't that hard to start. It takes more amps than a compact with a four cylinder but not nearly as much as a diesel truck.

I have had 8 gauge cables fail to jump a compact Toyota pickup. Maybe it would work if you waited 10 to 15 minutes for the battery to charge some, but I used my short 4 gauge and it started right away. Fat cables are worth it.

 

Jump starting a gas engine is jump starting a gas engine, you dont need anything special, I dont.
Smaller cables will take longer to transfer power is all.
We dont even use huge jumper cables to jump start big diesels at work.

 

It depends on how you do the jump. If you want to hook them up and use the other car to charge the dead battery some, then you don't need large cable. #4 would do. Personally I don't like fooling around with that, I prefer to just hook them up, let the donor car run for a minute or so, shut off the donor car and start the dead car. So when I made my 20' (maybe 25') jumper cables I used #1 welding cable.
I keep them in one of these, , so they stay neat and don't take up so much room.

Just like with battery cables, the smaller the gauge the less voltage you get at the far end which mean less voltage at the starter so slower turning and detriment to the starter.