Depends a bit on what it is you imagine the purpose will be. If you just want it for its aesthetic appeal, then 1 inch will suffice. If you think you will be keeping sand burrowing/sleeping fish in this tank, then 2 to 3 inches is probably better. Although I have never run a DSB in my main display, I believe 4 inches is viewed as a minimum.
I run 2 1/2 inches of the special grade sand in my tank which seems to satisfy my wrasses, has an extensive population of worms, and doesn't appear to be generating any chemical nasties.