In other words, a place to allow roof drainage and other storm water to slowly seep into the ground. It's a good idea, and a requirement in many areas. Such systems help prevent overloads to combined sanitary and storm sewers. They are also very useful in reducing the impact of development on our ever-worsening flooding problems. James' question is about the size of the damn thing, he didn't ask for a lecture about responsible development practices.
Sandy soil can absorb more water so it requires a smaller pit.
Clay and hardpan absorb very little water and they require a larger pit. So, I would suggest a minimum sized French drain pit of 8 cu. I would also suggest that you provide an overflow to the French drain, so any extra water can be controlled. Jacques A - Laminated glass has little value in terms of sound dampening qualities. Certain types of double and triple insulated glass have some special sound attenuation qualities.