How much chicken (by weight) should I be shopping for? In my experience, thighs can come in a wide range of sizes

It's your call and the decision should be based on your dinner guests' appetite.

If I were cooking this for immediate family, I would invariably go for smaller chicken thighs since none of them are huge carnivores who desire ginormous portions of animal protein. Admittedly we are part of the minority of Americans.

Ideally the thighs would be equally sized for even cooking. If you get a mix of sizes, consider putting the smaller ones in one baking dish and the larger ones in another and bake the latter longer.

You really don't want to serve half a huge thigh to accommodate someone with a smaller appetite, it'll wreck the crust.

Best of luck.

If buying boneless chicken thighs, get about half lb raw per person. As the recipe is to serve 6, get about 3 lb total.
If getting skin-on, bone-in thighs, figure about 3/4 to 1 lb raw per person, or 4 1/2 to 6 lb total for the recipe.
These figures come from a couple sites (the kitchen, my recipes) but also reflect general guidelines for estimating raw meat per portion (they allow for shrinkage during cooking, weight of skin and bones, average portions).
Adjust up or down if you know your guests or household appetites are bigger or smaller.