Well, usually in the basement you need high girth systems. You should aim for at least 3-4cm's for maximum effect, have a local store or specialist check it out for you.
Any plumbers around that could help you inform your decision. I'm sure they know the appropriate diameter for the piping downstairs. I'm thinkin a couple inches, but that may be a bit much.
I think the national board of construction and measurments should have some lists and documentation, I'm sure if you ask for it they'll send it to you for free.