25mm is good for about 170A - however it depends on a lot of factors, for example a 60mm cross section cable is good for about 400A - should start a truck, but make them too long and they could become no better than a 40mm cross section cable half the length.
Personally if I was making jumper cables I would use the heaviest cable I could lay my hands on, and make them as short as possible while being long enough to be useful.
I am going to be mounting my Anderson connector in my boot near the back door, or on the front bumper tucked away somewhere. I am going to buy 3 of them, one to mount to the vehicle, 1 to go on the end of jump cables and 1 to go on the end of a power distribution unit to run transfer pumps, inverters etc. While in Morocco need to do this a few times after letting pressures down due to differing terrain and don't want to be always last to finish.
For reference this goes roughly:
16mm sq 110A
25mm sq 170A
40mm sq 240A
60mm sq 415A
Now these figures then start to reduce as the cable length increases, so take your 25mm cable, after 3 meters at the full 170A load delivering 14V from the booster car, only 13.33 volts will make it there.
If you did the same job with 40mm sq cable, 13.6V would make it to the the other end.
Now my Bosch starter motor was rated at 2.2Kw, so 2200W at battery voltage say 12V is 183A so if you were to offer me a boost with your engine running, battery voltage up a bit (alternator cannot keep up with boosting demand so it will not be 14.4V while giving a boost) say 12.5V then the current drawn would be 176A - if the engine was in good fettle on the flat battery car, and the engine on the boost car was held at fast tickover for 5 minutes while connected to the dead battery 25mm sq leads should cope no bother, they might get warm on a long crank, but then again, so do most factory fitted battery cables, and this isn't necessarily an issue, as the insulation will probably be good up to about 80-90° C. As long as you don't cook them you will be fine.
