There isn't really a definitive answer to this question since casserole dishes come in all sorts of shapes and sizes. However, if you're looking for a good all-purpose casserole dish, something in the 3-4 quart range is probably a good bet.
Stainless steel casseroles are durable and easy to clean, so they're always a popular choice. Thermosteel casseroles are also very popular these days because they're designed to keep food hot or cold for hours at a time. And of course, there's always the classic Pyrex glass casserole dish - perfect for both baking and serving.