How do I list these elements according to their radius?
We have these elements:(just for ease, I'm naming them, you can just avoid it)


  $$
\begin{array}\\
Z=33&amp;\text{Arsenic}&amp;\ce{As}\\ 
Z=37&amp;\text{Rubidium}&amp;\ce{Rb}\\ 
Z=9&amp;\text{Fluorine}&amp;\ce{F}\\
Z=31&amp;\text{Galium}&amp;\ce{Ga}
\end{array}
$$


Now, let's see the periodic table (the useful region, thanks to Martin) :
$$
\begin{array}{ll|cccccccccccccccccc}
&amp;\text{Gr.} &amp;1 &amp;2 &amp;13  &amp;15  &amp;17 \\
\text{Per.}&amp;\\\hline
2&amp;&amp;. &amp; .&amp; .&amp; .&amp; \color{red}{\ce{\tiny^9F}}\\
4&amp;&amp; .&amp; .&amp;  \color{red}{\large\ce{^{31}Ga}}&amp; \color{red}{\small\ce{^{33}As}}&amp; .\\
\hline
5&amp;&amp; \color{red}{\huge\ce{^{37}Rb}}\\
\end{array}\\
$$

Now consider this:


While going down or towards left, size increases. Equivalent, is to say the size is larger towards bottom left corner


It wouldn't be now difficult to conclude that size is as follows:
$$37&gt;31&gt;33&gt;9$$