According to this
the average size of a string should be somewhere near the length scale of quantum gravity, called the Planck length, which is about $10^{-33}$
As far as I understand string theory - which is not very far - D-branes are extended objects on which open strings both begin and end.
As they are extended objects what are the size of their extensions? Are they of the order planck length squared, cubed etc depending on the dimension of the D-brane in question? I haven't come across anything that explicitly says that this is the case.