Yes, it is bad for eyes if you stare at the sun. Especially in intensive sun, most of people can't stare at the sun because they can't bear the light and heat from the sun, As for shrink the size of iris. It may not happen. but the sun can make you hard to open eyes that you may have to squint your eyes if you want to see the sun
Personally, it may not cause shrink the size of iris. But staring at the sun may cause the pupil dialated because most of people have to squint their eyes to focus the sun. IOn fact, it is really difficult to stare at the sun unless you wear sunglasses or polarized sunglasses. That because the brilight light usually make people can't open the eyes. Anyway, it is not good for eyes to stare at the sun.