Raised beds can be made from different materials. The more expensive a material is, the longer it will last. Cheap untreated wooden planks will last for a couple of seasons, when used in raised bed designs. It is cheap to get hold of, but will rot when it is in contact with soil for a long period. With wooden planks it’s possible to build as high as you want, for example waist high, if the structure is properly strengthened. Then there are the more expensive types of wood which will last several years despite being in contact with damp soil. If you don’t care about the looks of the raised bed and prefer long life above all, then poured concrete is the best material. It can be formed exactly as you want it to be, and it will last for a lifetime. If you don’t want to pour it yourself you can use concrete slabs, and assemble a raised bed.
The width of a raised bed is limited by the ability you have to reach the middle of the bed. It’s good to have a wide bed but if it’s to wide it’s impossible to remove weeds near the middle by hand. One of the principles of using raised bed designs is to grow the plants so close together that you don’t need tools. When the plants are growing shoulder to shoulder it’s very hard for the weeds to get any sunlight, and they’ll die. A raised bed width could for example be 1 m or 1.2 m (3.3 ft or 4.3 ft), and the length could be the same, or twice the width, i.e. 2 m or 2.4 m (6.6 ft or 7.9 ft).