I need a world where men are 3'5" and women are 6'5".
The small males with have large penises, be ~3'5" but completely proportional, cute-handsome faces and insane reaction speeds and incredibly athletic. Their bones will be very dense.
The women will be 6'5" with huge hips and nice big supple wombs, big breasts and pretty faces. They will naturally superovulate and produce small litters when impregnated.
Both of the sexes will be incredibly intelligent, of course.
No other form of humans will be able to compete with them. They expand significantly faster for fewer resources and they have a huge advantage in technologically advanced warfare and labor. Any nation that goes against them will be out competed and destroyed.