Containers should complement the style of your home and the appearance of the plants you intend to add to them. As you select containers, keep in mind the mature size of the plants that will inhabit them.
Poured-concrete containers with the look of stone are popular and well suited to a formal setting. They're very heavy, so once planted, they should be considered a permanent part of the landscape.
Terra cotta comes in a huge selection of shapes and sizes. Over time these pots acquire a beautiful aged look . . . as clay pots age, algae appears on the outer surface, as well as mineral salts from fertilizer and water. Some people like the appearance of an old clay pot; others find it unsightly. The disadvantages of terra-cotta pots are that they allow the soil to dry quickly, which means plants will need more frequent watering, and in cold climates they may crack as soil expands and contracts.
Some of today's plastic containers resemble terra cotta. They're lighter in weight but will never attain the attractive aged, mossy look of the real thing. Plastic containers retain moisture better than clay, which is an advantage in hot or dry climates but a disadvantage if you tend to overwater plants. Plastic pots are less expensive and readily available in many designs. They're lighter than clay pots and may topple if plants are top-heavy.
Fiberglass containers are lightweight and long-lasting and may have the "aged look" built in. They can be made to look like terra-cotta pots, wooden containers or even bronze or copper containers, with a finish that resembles those metals' natural patina. They won't crack if left outside for the winter.
Wood is an excellent traditional choice and comes in a variety of styles, from redwood buckets to upright square boxes with feet. Wood dries out more quickly than other materials and may not last as long. Some plastic planters look a lot like wood and last practically forever.
The standard pot is as wide as it is tall, so one with a 6" diameter is generally about 6" tall. A standard-shaped pot is a good choice for most plants. Make sure that all containers have drainage holes.
Dark-colored pots absorb more heat than lighter ones; roots stay cooler in lighter pots. If you live in a cool climate and want to grow cacti, choose a dark pot; if you live in a warm climate and want to grow tender annuals, select a light-colored container.
Happy gardening :)