Numbers

My examples all use the group size fixed at 70, but that is only for illustrative puposes.

Any number in the range around 50-100 would do. 

And in fact there is no reason whatever that each group should have exactly the same number of members.  It must be obvious that as the population changes, a group may have to expand or contract, and at some point it will become obvious that it should gain or lose some members to other groups.  Even splitting in two should be possible.  None of this should affect very much the governmental system.

It interesting to consider that a member has more 'clout' in a smaller group, since he has less inertia from the other members to overcome.  So in a sense smaller groups are more 'powerful' than big ones, just the reverse of the case in most current government systems.

Play with the numbers - links to a Google spreadsheet