Complex sentence: The difference of the major and minor semi-axes is about 21 km and as fraction of the semi-major axis it equals the flattening; on a computer the ellipsoid could be sized as 300px by 299px.
Split into simpler sentences: The difference of the major and minor semi-axes is approximately 21 km and as fraction of the semi-major axis it is given by the flattening. Thus the representation of the ellipsoid on a computer could be sized as 300px by 299px.