$\mathrm{Mg},{ \mathrm{Mg}}^{2+}, \mathrm{Al}$ and ${\mathrm{Al}}^{3+}$ are arranged in decreasing order of size $1>2>3>4$. Species which are present at 1 and 4 position respectively are :

Atomic radius iis decrease through period. One of the isoelectronic ions will have a smaller radius if it has a higher positive nuclear charge. therefor $\mathrm{Mg}$ is the greatest species, where as ${\mathrm{Al}}^{3+}$ is the smallest.