Answer: Raised beds are typically built to dimensions of stock lumber, such as 4 by 8 feet for economy sake. The overall dimensions and the height are determined by personal preference, however there are some factors to take into account. (A common height is only about eight to twelve inches due to the construction difficulties of going higher.)
The cubic space within the box must be filled with soil mix, and that can be a considerable expense if you are purchasing the fill. Next, the higher the bed, the faster it will drain and dry out, so you will need more frequent watering. The narrower the bed, also the more frequent the watering as there is more soil closer to the edges where it dries out faster. Next, the taller the bed the stronger the sides need to be due to the weight and pressure, and the taller the bed the stronger, deeper set, and closer together your support posts will need to be. Local building code specifications for retaining walls would be a good start in terms of determining what is required for construction design; you might also want to consult with a contractor.
In terms of wood, treated wood is no longer routinely recommended for vegetable garden sides; redwood or cypress or cedar are naturally long lasting, there are also some recycled plastic products turned into a very long lasting wood substitute (eg Trex); you can also use cinder blocks or even regular pine boards will last a number of years.
I understand that bending is a concern for you, but given the above considerations you may find that it is preferable to use a scooter or kneeling bench or other means of sitting closer to the ground so that the bed does not need to be quite so high. You could also make a wider edge or bench seat on the sides of the bed so that you can sit on it while you work. I hope this gives you some ideas, all the best with your project!
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