The ideal height for a raised bed is a matter of preference for gardeners. Considerations include the cost of raised beds and therefore the amount of soil needed, the condition of the soil beneath the bed, soil depth requirements for the intended crop, and especially important to mature gardeners, how much bending over you want to do.
Always allow for space between gardens for you and possibly your barrow to get through. Thanks Judith for the great picture.
Why not try setting up a Wicking bed. Self watering for up-to 4 weeks and you save your nutrients and the environment.