What gauge is the crossover cable?

Measure it, and look it up on this chart: https://en.wikipedia.org/wiki/American_wire_gauge

I used a 2/00, it's bigger but worked fine

I also went bigger. The stock cable is bare minimum for the DC flow.

The stock cable is plenty for any conceivable draw with the factory or direct-replacement batteries, starter, alternator, & other loads.

That's a funny joke

It's a 4 gauge on my '04 and that is definitely way too small IMO. I ran a 2/0 when doing my replacement. Of course I also replaced all of the factory battery leads with bigger/better.

Factory gets by with the bare minimum, the bean counters won't let them even if it is the consumer that pays in the end.

When I replaced all of my cables I went larger, less wear and strain on them than using the factory gauge especially if you add any aftermarket electrical.