I'm starting a tournament, but I don't see a "Max Group Size" option.  Is this a feature only available to premium members of a more elite echelon than myself?  If so, does this mean that--in a 10 player tournament with 2 games per opponent--we'll each begin with 18 games?  That seems a bit overwhelming.

Can anyone explain my inability to limit group sizes, or at least offer suggestions for how to run a manageable tournament?

See here http://www.chess.com/tournaments/help.html

Then "How do different membership levels (free vs. premium) affect tournaments?"

Silver and Gold members cannot create multi-round tournaments.

You can set one game at a time rather than two, so 9 simultaneous games not 18.