What Size of Containers for Herbs?

I want to grow all of my herbs in containers. What diameter and depth would you suggest as a good rule of thumb? I want to buy them in advance of an order.

There is no hard and fast rule about the container size. Most herbs do not do well for long in containers less than a 10 cm (4 inch) across. They do better in a 15 cm (6") pot or even a 20 cm (8 inch) pot. The depth should be at least 10-15 cm (4-6 inches) deep. Some herbs will need to go into an even bigger pot as they grow and become pot-bound, or in some cases they can be divided and repotted if you don't have the space for a large pot.
Larger containers such as windowboxes, strawberry jars and bowls can work very well for herbs too with several varieties planted together. That way, the plants have more room to send out roots which allows them to grow better above ground.
Be careful not to plant in too large a container, though, because you could have problems with watering. If the plants are too small you can easily end up watering too much, causing root rot and the eventual demise of the plant.
A great book on container growing of herbs is "Little Herb Gardens", available from Richters.