I've found over the years that raised beds of 4 x 16 feet (1.2 x 4.9m) work very well. It's important not to go beyond 4' in width (unless you have orangutang arms!).
There are pros and cons when choosing materials. Concrete block borders are great & solid, but very permanent! Wood is easy, aesthetically pleasing and easier to change, but unlike concrete it rots eventually.
Putting posts in the corners is a good idea, although I prefer to use alkathene hoops (blue water pipe) for the job. If you put saddle brackets on the outside of your borders, then you can slot the pipes into them to keep them rigid, for draping over the netting or possibly plastic sheeting (like a mini polytunnel).
The height of the beds is up to you. Some, especially disabled gardeners have them at heights of around a yard (1m) so there's no need for bending- but they take a hell of a lot of filling!