I am at a huge conference with two thousand participants. The forty-odd parallel sessions are going on in four different buildings. Even if you want to meet someone on the participant list there is no chance. People stay at dozens of different hotels. I have only met the people I've come here with. You don't make new acquaintances at lunchtime since there are no arranged lunches. I didn't go to the reception yesterday because I am scared of crowds. The collegue who did go said the food was excellent, and plenty of wine. At least he got something for the incredible conference fee.
I have only been to a conference this size once, which was the American Comparative Literature Association. We were a group of twelve who met for two hours each day for three days running. I never went to any other sessions. It was one of the best conferences I've been to.
Nest week I am running a conference at Homerton. There will be seventy participants. Everybody will be staying in the same place and have meals together. Some people will find it boring and want to go and have dinners on their own. I haven't been able to avoid parallel sessions, since I need to accomodate all papers. People will want to be at two places at the same time.
I have been to conferences of this size often, and one thing is that you feel you have met everyone. Or at least have a chance of meeting everyone (you still have the option of staying in your room for the whole conference). There is plenty of time for informal talk. You meet people you have not met before.
Some of these conferences were good and some weren't.
I guess size doesn't matter.