How do you write the elements #Se, Rb, S# in decreasing atomic size?

To understand the trend for atomic radius, you really only need to understand the trend for electronegativity. Electronegativity is the ability of an atom to attract electrons to itself, and its trend goes as shown in the diagram below:

Why is this though? Well consider what's going on at the atomic level. As you move right across the periodic table, you're adding more and more electrons to the energy level, but you're also adding more protons. This means that all the electrons experience a stronger pull towards the nucleus, something known as #Z_(eff)#. As a result, we see that Florine is the most electronegative element.

What happens as you move down the periodic table though? As you do that, you're adding electrons to higher energy levels. This means that these electrons will have much more energy than their counterparts in lower energy levels, and hence will be able to resist the pull of the protons better.

How does this relate to atomic radii? Well, consider what would happen to the size of the atom if your electrons were pulled closer in towards the nucleus: it would be smaller. As a result, the most electronegative atom would have the smallest radius, and the least electronegative atom would have the largest radius.

Hence, your listed elements in decreasing atomic size would be Rb, Se, S.

Hope that helped :)