I use masonry mixing tubs for my sandpits. They are 3 feet by 2 feet and 8 inches deep. They can be filled with either playground sand or washed multipurpose sand. All of these items can be found at your local hardware store.
The bottom of most Diamondback terrapins nests are around 6 to 7 inches deep. The deepest nest I have seen was actually 10 inches but that has proven to be a real rarity. I suggest having your nesting area a minimum of 7 inches deep with sand. If your nest area is less than 7 inches deep they might still nest but they will be more likely to dig more test holes and carry the eggs longer before deciding to finally lay them.