As you have suggested if the tank is long and narrow or very tall it will have a low surface area, putting restrictions on the size of the fish and how many you can stock. Larger fish will usually have a higher metabolic rate which means a greater demand for dissolved oxygen, this in turn means they need a larger surface area. You'll be surprised at the difference in oxygen demands between a fish that grows to an inch in length and a fish that can grow to twice that length.