I guess I should have explaned with a little more detail.
From the research I have done and from speaking to many people a majority of reef keepers have there sand beds at around 3 to 4 inches.
Here is a quote from one of the experts in the field of a DSB.
Ronald L. Shimek, Ph. D
"In my 45 gallon reef tank, the sand bed averages about 4 inches deep"
There are some people also that are very succesful with sand beds deeper the 4 inches and some that are less then an inch. It all depends on the grain size of the sand.
Robert Stark
"One inch of sugar-size oolitic aragonite with an average particle size of 0.5 millimeter carries approximately the same surface area as 4 inches of 2 millimeter size sand"
Just like almost everything else in this hobby what works for some may not work for others. Do some research and make logical choices and everything should go smoothly.
Here is a quote from another web site that sums it up nicely I think.
"Using a coarse substrate, the bed would have to be very deep, perhaps six or more inches, to create sufficient denitrification. Finer substrates can have this effect at 4-5 inches. Ultrafine substrates can even accomplish denitrification in under an inch of depth. The difficulty in setting up a successful deep sand bed arises from the lack of proven formulas for success. Finding the right depth and grain size are more a process of trial and error than calculation."