Sizing depends on the type of nappy really. Some are made small and some are bigger. A bit like kids clothes from different shops iyswim. Is your daughter in nappies during day?
Please feel free to ask as many questions as you like. Also look through posts in the nappy sections and you will pick up useful info there. I think the best way to decide which nappies you like/are the right size is just to try some. Thats what the classifieds are for!! You can pick up some real bargains and if you find you don't like the nappy you can sell it on
But just to warn you what you finf works with your daughter may not work with a newborn...its all trial and error really and having loads of fun on the way...