I think for WordPress or any other platform you should have an ad just below the title of the post. The size should be 336 x 280. These are the best. Now you can put a 160 x 600 sky scrapper in the sidebar. These days Google Adsense has come with a 300 x 600 large sky scrapper that you can put that on the sidebar also instead of 160 x 600 .
I believe best place can be determinate by placing ads to different places and checking where it gets most % of clicks.