I’ve never organised anything as big as the conferences you mean, but even so organising T-shirts is a pain. Do you ask everyone in advance what size they want? So much extra faff and the size guides are often not that helpful. Do you get loads of extras so that you probably have the right size for most people? Or do you get the right number and accept that the person at the back of the queue will just have to make do? Do you get more larger sizes because too big is better than too small, but then accept that small people probably won’t have something that fits well? So much cost and effort for something that has so little impact at the end of the day.