the shape of the softboxe will show on the reflection surface of your subject, eg for a human, that will be the shape of catchlight.
to get effective softness of light, the size of the softbox should be the same size as your subject, eg, full length portrait using a 6ft softbox,
and also the optimum distant between the softbox and subject, the distance should be the same as the diagonal of your softbox. eg, a 3ft x 3ft softbox should place about 4ft away from your subject.
hope this help.