Our club, Hogtown Brewers in Gainesville FL, has about 100 members. Most meetings average 25-30 with a max of about 40. We meet at member's homes, beginning each meeting with a pot-luck dinner. We haven't had a problem yet, but have about maxed the possible size for meeting in members homes. Not sure where we'll go if/when membership & meeting attendance grows beyond our current limitation.
Our meetings start with the potluck dinner, then we do Homebrew Show n Tell, then any necessary meeting business/announcements, followed by the presentation, and then just partying. We do the vast majority of club business at separate exec meetings (to which all members are welcome - though it's usually the same group 10-12 comprised of the officers and the few others that carry the weight of making things happen. We went to this format a number of years ago because of lack of interest/attention of many members during the meetings. Partying is easy (beer, homebrewers, what else do you expect?), but the rest takes effort. So far our format has been working well, though some meetings do get a bit scattered during the presentation.