the deeper the beds the better but I have two different heights. The 1st are made of 6" boards & have been providing excellent veg for over 16 years. The 2nd are made up of 10" builders boards I put together two years ago. It really is down to how much compost you put into the soil at the end of the day. Have a look at my photo's to see the two different sizes ( page 7 & 8 ) . Good luck.

I would agree the deeper the better, but having said that our couple of very amateur raised beds are hardly raised at all by now, we need to refresh them with more soil and compost to raise the level of the earth. In spite of that they do well because of the demarcated area of ground which does not get walked on and compacted and can be very easily weeded.

Make sure you keep them well watered as they dry out far quicker than a normal bed.