Chlorine (up) is better oxidizing agent than bromine (down). Magnesium (up) is better oxidizing agent than Calcium (down). So, generally, oxidizing power decrease as you go down the periodic table. But there are exceptions.
You can look at the electronegativity values, the more electronegative elements are on the top and they have a greater oxidizing power. You can also look at the electron affinity values, they decrease going from top to bottom, and this also helps explain oxidizing powers.