Could diameter have an effect on the magnitude of a star?

Yes, the diameter of a star can significantly affect its magnitude. Larger stars tend to have greater luminosity, which can lead to a brighter apparent magnitude when viewed from Earth. This is because a star's magnitude is influenced by both its intrinsic brightness (luminosity) and its distance from the observer. Therefore, a star with a larger diameter, assuming it is also hotter and more luminous, would generally appear brighter in the night sky.