Experts and BHG readers answer.
If the lavender will be in a pot by itself, you'll want to use a pot somewhere between 12 inches and about 24 inches in diameter. You may want to start with a smaller pot, and gradually move the plant up to a larger size as the plant grows. The advantage of using a larger pot, is that the plant will grow larger and require less frequent watering with a larger pot. However, if you place it in a huge pot to begin with, the roots won't readily fill the container, and part of the soil will be wet while the root zone has already dried out.