How do I construct a raised bed? What is a good size for a raised bed planter?

FAQ
Question: 

How do I construct a raised bed? What is a good size for a raised bed planter?

Answer: 

Raised beds may be permanent or temporary structures. Temporary raised beds are shallow beds (6 inches or less in height) constructed within the existing garden. After the garden is tilled in the spring, the loose soil is raked into raised beds. Permanent beds are usually higher than 6 inches in height, have side supports, and are built to last for many years.

The walls of permanent beds can be constructed of decay-resistant wood (such as cedar or redwood), concrete block, rock, or brick. Treated wood can also be used, but depending on the treatment, the inside may need to be lined with woven geotextile fabrics that will allow water to drain through but separate the soil/plant roots from absorbing potentially toxic chemicals.

While there are few rules that must be followed, some guidelines should be kept in mind when planning and constructing raised beds. Raised beds should be constructed so an individual can reach everything in the bed without stepping into it. If a raised bed is placed against a building, fence, etc., making it accessible from only one side, the maximum width should be 1 1/2 to 2 feet (i.e., approximately arm’s reach). Beds accessible on both sides can be 3 to 4 feet wide. The length of the bed is determined by space limitations, personal preference, and convenience. If constructing several raised beds, the pathways between adjacent beds should be wide enough to accommodate garden equipment (e.g., a wheel barrow or cart; ~3’ wide).