Size: 5–7 quarts. Smaller than 5 and you constrain the oven spring. Larger than 7 and the dough spreads instead of rising up.
Material: enameled cast iron is easiest. Bare cast iron works fine but needs seasoning. Ceramic is beautiful and fragile.
Shape: oval for batards, round for boules. If you can only buy one, get round — it's more versatile in the kitchen beyond bread.
Brand: Lodge is honest and affordable at $60. Le Creuset is beautiful and lasts forever at $300. Both bake identical bread.