You’ve really put your finger on an octopot mystery there @kevinmalone. 
The original octopots have big reservoirs and big fabric bags and they grow really big plants.
The micro-octopots have small reservoirs and smaller fabric bags, and they also grow really big plants.
A few folks have experimented with really small pots and also had good results (@SaintAliasKnife!). So I think the best answer is that nobody knows!
One thing to consider is that a properly setup octopot develops two root systems, one in the reservoir and another that fills the fabric bag. The relationship between those two root systems and the optimal balance between them is not completely understood.
My own preference is a three gallon fabric pot that is sized such that the upper root system almost fills the bag with roots. That approximately balances the size of the two root balls and I’m just guessing that is close to optimal.
There are plenty of growers who have more experience than I do with octopot growing and I’d suggest that you put this question out there to tap into their knowledge. @syn, @Papalag, @firehead, @Jetdro, @Floyd, @Enjoi802, @SaintAliasKnife, just to name a few.
Here’s a good starting point for more octo info if you haven’t already bookmarked it: