Correct order of metallic radius for alkali metals should be:

The correct order of metallic radius for alkali metals is $\mathrm{Li}<\mathrm{Na}<\mathrm{K}<\mathrm{Rb}<\mathrm{Cs}$.Because down the group size increases.

There is more space between the nucleus and the outermost orbital as one moves down a group because there are more energy levels (n). A greater atomic radius is the outcome of this.