I don't even think of using pots smaller than six inches, because of our summer heat. If containers will receive full sun, make sure they are 16-20 inches in diameter. Smaller sizes like 12-16 inches will work if afternoons are partially shaded. My longest container success is a rusty metal wheelbarrow overflowing with rosemary, thyme and ivy. It gets about six hours of morning sun and shade the rest of the day.
Roots frequently accumulate next to container walls.
To help keep them cool in the summer try double potting. Put potting soil between the pots as an insulator. If you use decorative ceramic containers, be sure they have drainage holes, otherwise use them as a decorative cover to hide a well draining but unattractive container. Plant roots function like a water pump, taking minerals and nutrients to every part of the plant. Roots die in both situations.