The most stable electron configuration ends with p⁶.
Ne, Ar, Kr, Xe all are very stable and end with p⁶.

There is extra stability associated with p³ (the half filled p)

Filled s (s²) also has some extra stability.

d10 and d5 have some extra stability.  The stabilities follow this trend:

p⁶ >> s² = p³ > d¹⁰ > d⁵

This influences the properties across the periodic chart.

Trends in size:

As one goes down the chart, there are more electrons and therefore one would expect a larger atom.

However, going across the chart, one get closer to the more stable p⁶.  This makes the atom "tighter" and the atoms are smaller.

The trend in size is therefore from Fr to F  as large to small.

Examples:    S is bigger than F
                    Se is bigger than F
                    Se is bigger than S
                    O is bigger than F

How about O and Cl.  Can't tell from the diagonal.  One has to look that one up.

Very few exception to the overall diagonal trend for atomic size.