Dear Sara and Dorie,
Thanks, Sara, for getting us started. Well, here's my attempt to define the cookie: I'd say that it's handheld (although I've seen a few that tip the scales in the other direction); it's something meant to be consumed in a few bites; and it absolutely, positively has to be round. OK, I just made that last one up. Of course, you're right to point out that there are plenty of minisized treats and bars out there, like Anita Chu's Viennese almond crescents, which put my theory in question. Still, I would say that, for me, a good cookie should be big enough for at least three bites. Maybe Dorie can be the brain on that, and I'll be the brawn?
I agree with your explanation of why the American cookie is a chunky one. We Americans are "customizers." If you go to any restaurant in America, it's practically de rigueur to ask whether the chef can change everything on the menu. And we also take a "more is better" approach. Most folks feel a restaurant is a good value if there's a lot of food on the plate.
As for those thin, nutty cookies you mentioned, you probably won't find them at bakeries, since they're a lot of work to roll, cut, and bake. And they're fragile, meaning there's going to be a certain amount of breakage. (When I worked in a restaurant kitchen, I never had trouble getting rid of broken cookies, due to the steady swarm of hungry chefs milling around the pastry department at all hours.) Cookies take a lot of time, and anything fancy or small is going to be more costly and time-consuming to produce. That's probably the appeal of larger cookies in bakeries and with home cooks. On that note, I couldn't eat a 9-inch cookie, but if a cookie is good, I want more than one tiny bite of it. So a happy medium is appreciated.
Here in Paris, if I buy cookies, I prefer the kind that are difficult to make at home (especially if your kitchen is postage-stamp-sized, as is mine). I often go for macarons, which, like baguettes and croissants, are readily available and inexpensive. When I'm back in the States, if I'm in the supermarket, I can't resist Mallomars: big puffs of marshmallow sitting on a graham-cracker-like base, covered with the thinnest layer of dark chocolate. Those, and sugar wafers, remind me a lot of my childhood. But of all the store-bought cookies, HeyDays were the best—long wafers covered with caramel and dark chocolate, completely blanketed with toasted nuts. Perhaps they disappeared from the marketplace since they fell in that dubious area between cookie and candy bar: You're entitled to eat a bag of cookies, but few folks feel comfortable plowing through a bag of candy bars!