The atomic radii decreases as you go from left to right on the period. This is a result of an increase in the effective nuclear charge felt by the valence electrons of the corresponding atoms. This trend is very well documented and experimentally determined for all of the elements; except neon. Neon appears to be larger than fluorine and a few other elements in this period, but that is because there are no compounds of neon and so, the atomic size that is determined is its Van der Waals radius, which is an upper limit but not the precise size of the atom.
Atomic Radii Of Elements
the change in radii is a decrease because of an increase in protons. but if you were determining the change in radii going down a column of the periodic table the radii would increase in size because there are electrons in new energy levels further away from the nucleus as you go down.