Since you're only concerned with the "thickness" of the wire and not the brand, I'll presume that you don't believe in brands.

Given your requirement of the run at 6 ft, either 16ga or 12ga should do. However, the price difference of the two isn't that great, I'll suggest to go with 12 ga. Check out Ultralink Challenger 12 ga. This is a Canadian company and if you're nice to the dealer, you could get them for about $1/ft.

Regards,
HT_Man