In general, as you go across a row on the periodic table the atomic radius gets smaller. The reason for this is that the nucleus is getting larger and the densely packed, postively charged nucleus attracts the electrons toward itself more and more strongly.
The exception to this is the noble gases in group 18. They are always much larger than the corresponding element in group 17.
Note also, that the transition metals generally have about the same size atomic radius.
In a row in the periodic table, as the atomic number increases, the atomic radius generally Decreases .
This is because as the amount of electrons increase so does the force of the nucleus.