Very approximately (mine are a bit old & crumpled) the new born are 11x15 inches & the next size up are 13x18 inches.  I bought mine a long time ago when there were only two sizes.  From what I can gather, there still are only 2 sizes if you're talking length & width, but you can get the 13x18 inch nappies in different thicknesses (not sure if it's the whole nappy or just the middle bit) for when they're different ages and weeing more/less often.  I've just stuck with what I've got & am managing fine, though my 13 month old may need boosters soon for night time.  Other than that, they're just fine.