The wrap length you'll need depends on the carrying method you choose as well as on your own size.
Size 2 is suitable for the "Cradle" or "Hip Carry" method, size 3 for the "Back Carry", size 4 for the "Kangaro Carry", size 5 for the "Cross Ccarry" and sizes 6 or 7 for the "Wrap Cross Carry". That means, if your own body size is no more than 14 you can use Wrap Size 6 for all carrying methods. For body size 14 (GB) / 12 (US) we recommend Wrap Size 7.
Sizes and tying methods:
Sometimes the mother is much smaller than the father or another person who wants to use the wrap as well. In this case we recommend to choose a size that suits the mother. The taller person may not be able to tie ALL carries, especially the Wrap Cross Carry, but at least the mother won't have to handle a wrap that is far too long for her.