I like beds that are about eighteen inches high, but you need to consider your own site and what you are trying to accomplish. Examine the slope of your ground. The height of your beds may vary from side to side depending on the lay of the land. When all is said and done, you will want a level planting field. A flat surface reduces the opportunity for erosion and makes it easier to provide consistent moisture to your plants.
Shape is another consideration.
A large square or rectangular box is generally the easiest to build. Their sharp corners can be hazardous, though. If you have children playing in your yard, you may wish to soften corners by rounding them off. Also, these beds can have a rigid, formal countenance that may be a little too Spartan for some tastes. Follow the contours of the land with gentle curves or create your own configuration.