what gauge wire is the biggest electric wire?
I think the largest normal production cable is 2000 mcm . The ampacity is around 1500 amps in air, less in conductors; depending on what service it is in. A 4/0 wire is rated in the neighborhood of 400 amps. A good project for you is to look up the meaning of mcm. Hint - circular mils.
General English riding, yes. Dressage, no. The copper and the rollers make this bit illegal if you plan on showing Dressage, otherwise it's fine.
Yup, a D-ring snaffle is fine to use for english.
To add to what BillRussell said, 0000 cable is about the max practical size a person can carry, hauling a 50 foot length of four-ball is very much a work-out. In the Navy we had to move 400 amp shore power leads that consisted of about a dozen 0000 leads in parrallel, and to carry these on our shoulders we were practically in contact with each other: Copper gets heavy fast.
0000 AWG wire is one of the largest and fairly easy to get, but in reality there is no upper limit. For very heavy current, beyond a certain limit defined by your local wire code, busbars are preferred. These are solid bars of copper or aluminum. .
