raised bed

I have shifted loads of topsoil from my garden to my plot. i want to make a raised bed. How deep/high should they be.  Thanks

My raised bed is approx 20 cms (8") high, and are 120 x 120 cms (4 ft x 4 ft) square. I have peas, Onions, Spring Onions and Radish in at the moment....although the Radish are going down fast due to me eating them - Delish !  

ours are 1-2 ft high (countering the slope on our plot) and about 2m x 2m which is actually too big because you can't comfortably reach the middle