I have similar raised beds. Mine are 4'x8'. With beds the size of mine, I recommend a brace across the top connecting the centers of the longer sides as well as stakes driven at least 12" into the ground at bottom at the centers of the longer sides to keep them from bowing out. Mine are 26" inches tall (the width of the roofing panels) and filled with 20"-24" of soil (I keep adding and it keeps settling). I placed branches, brush trimmings, logs, etc. in the bottom of some of the beds, which adds a bit f hügelkultur to them also. The only problem I've had with them is they dry out faster because of the added height - gravity and heat work against moisture retention. If we have a week or more without rain, I have to give the beds a good soaking with the hose. A drip irrigation system would be great! Maybe one day...