I expect people to have a better idea of group sizes than normal distributions, but the worry is with 'chosen uniformly at random,' since people tend to be in IQ bubbles. If you can think of 100 people you know dumber than you, and 10 people you know smarter than you, then you might say "well, that implies a group size of about 10," without realizing that half of those people you know from graduate school where the average IQ is in the top percentile.

Meh. If I was asked that question, I'd try to guesstimate how many people smarter than me there are in my town and divide by its population.