So notice that we have two energy levels, one S and two S for the lithium, while the sodium has not two, but three energy levels. One S, two F and three S.
Now, this addition of the three S means that our atom will grow in size, will enlarge. Where this guy, the atomos guy, is the three F. Shell. So that means when we move one down to potassium, potassium will have a four S. So potassium will be even larger than sodium and definitely larger than lithium. And that's exactly what we see. In other words, as we go down a group, our atomic radius tends to grow in size, while as we go across the period, our atomic radius tends to decrease because our effective nuclear charge of our atom tends to increase.