Atomic Size: Atomic Size is based on radius of an atom. As you go down the periods the atom gets larger. Because you measure the distance between the nucleus. As you go left to right with the groups, the atom gets smaller. This can be tricky to because you can get some diagonal ones that the smaller number is the bigger atom.
From largest atom too smallest atoms in period 3: Na Al P S
From smallest atom to largest atom: Cl Br Fe Rb
From smallest atom to largest atom: F Ag Sr Cs
Ionization Energy: Energy required to remove electrons. Opposite the directions of Atomic Size. The farther down on the group, the easier it is. The farther right on the group, the harder it is