There is no specific answer, but different styles works with different niches. And you need to experiment will all those to find the best one in your case. Still in general the square type (W:300, W:336) can do a better job.
All Placements depends on Template and Content, I use 300x250,160x600, in my site. All banner must be place before top-half portion of site. If you have blog try to place banner in text content too.
for CPM no matter where you place the ads. But, while coming to the click, I'd suggest that to place on the top of the website like how msn or other top sites do.