6ga is pretty damn thick, imo. and will kill all but the finest binding posts as its so stiff and heavy.
i'd guess 10ga would be just about perfect. I'm happy w/my 6's though. good investment.
here are mine, sans Vamp spades (have them, but have been lazy). Mine are set for "normal mode", as my speakers only have two posts.
I had them in a bi-wire config for a while, and those really kicked ass. It was super easy w/this stuff. the metal braid consists of 19 other braids of 19 wires. you can untwist the entire thing, and re-twist it into two (one w/10 and one w/9 braids) leads at the speaker end, and leave it as is on the amp end.
this way, you only need those 6ga spades on one end, the amp.