Randy,
I just added sand to my new tank a few days ago...don't even have rock in yet! I went with a DSB and was told by a VERY GOOD SOURCE at our local reef only store that fine sand is best for a DSB. I bought enough sand to go 6", but when I started adding sand, I realized that 5" was plenty! So, I have 5" and it looks great. I went with the finest stuff they had, basically like sugar. I was told that finer is better for DSB. Good luck.