Which is smallest 1AU 1 light-second a star or a planet?

Planet by a long way.

They vary but in our solar system the largest is Jupiter and it's 66,854km across the middle.

A light second is the distance travelled by light in a second = 299,792.5km

A star can be lots of different sizes - our Sun is = 1,391,980km

An AU is the distance from the Earth to the Sun = 149,598,000 km