Bigger is always better, an adult pair of diamondback terrapins need at least a 70 gallon aquarium. I use 150 gallon stock tanks for my adults and I wish I had the space to get bigger tubs. I suggest keeping around 4 females and 2 males per 150 gallon tubs.
Stock tanks are great for breeding because they are normally deeper than aquariums. Most adult diamondback terrapins need the water depth to be at a minimum of 12 inches before they will mate. The Rubbermaid stock tanks that I use can be found at most feed stores for around $150.
Waterland Tubs are also a great setup for breeding. I really like the medium and large water enclosures for breeding.