Reddit mentions of Micro USB Cable, Tronsmart [3 Pack] 20AWG Durable Charging Cable for Nexus, LG, Motorola, Android Smartphones (Black, 6ft x 3)

Tronsmart is the only brand I've found that consistently works. I've only found one weak Tronsmart cable out of probably 30 (way better success rate than any others I've tried).

I have not seen any truly scientific tests, but this is on the right track.

From what I understand, most USB cables are 28awg, so finding one with larger wires (which would be a smaller awg number) will prevent the voltage from dropping too much over long distances. Something like this from Monoproce with 24 gauge power conductors should perform better, and this Tronsmart cable with 20awg conductors should be even better, but isn't available in anything greater than 6 ft.

The Anker PowerLine cables are good quality mainstream cables, but there seem to be some inconsistency in their marketing. Some diagrams show 20awg positive power conductors, but not equally sized ground conductors, and the product listings themselves don't quote any specific gauge - just that they're "designed with a thick wire gauge".

As for length, as I am sure you know, the rule is to use as short a cable as you can get away with.

Unfortunately I don't have any data or rules of thumb about how long is too long.