from what ive read about 6th you would take "normal" size port for those subs on a regular ported box and basicly shrink it by half - in the hopes to control sub suspension when playing under tuning. also whats your front chamber going to be tuned to ?
and are you chasing numbers or wanting to play a large bandwith ?
for numbers you can keep the rear tune closer to the front tune
for daily music and ground pounding you could tune the rear in the 25 to 30 range and the front somewhere 55 to 60hz and play a pretty strong bandwith from 20s up to 60s