Large enough size to prevent movement in high flow tanks causes detritus build up and is of little use to the critters we keep a sand bed for in the first place. Personally I find the large "chunky" substrate to look a bit odd, I like the look of sugar sized sand. I have a 50g. cube (24x24x24) with 2 RW-8's on full blast W1 setting and a Mag 5 for return. After much adjusting on the direction of flow I have the dunes down to a minimum.