My best performing is 300 x 250 within the post. I'd suggest you to create a channel for each ad placement so you can see the best performing and ditch the one that's not.
here is a best answer for u: http://support.google.com/adsense/bin/answer.py?hl=en&answer=17955&ctx=as2&rd=3
This all depends on your theme, you might want to do some split testing if your website is getting enough traffic. However I wouldnt recommend split testing unless you have at least over 100 uniques per day or so.
There is no easy answer for this, because what may work well for another persons niche site, may not work for you. It's a lot of trial and error really. Try different sizes and types each for short period and see how they perform and then choose the best performing one.
I think the best size is 300x250 but for the placement it depends on many factors from your website, you should try and test wich placement fits bets for your website and earns best. Do some split testings.
This is what Google reccomends > http://support.google.com/adsense/bin/answer.py?hl=en&answer=17955&ctx=as2&rd=3 I also use ads in post. Check any article at www.technouz.com - and they make me a lot of money If any newbie has a wordpress site that needs SEO and an advertising system set up, PM me.
just add your ads above the fold or below your post title, 300 x 250 is a good choice to implement it
Try to put your ads at the top of your article and heading, if possible, you can put an ad in front of logo, it converts good..
Try these :- Top of the post, below the post title - 336x280 Bottom of the post - 336x280 Sidebar - 300x250 You will surely feel increase in your adsense earnings.
header banner text/multimedia 728x,sidebar banner and in the featured or top post 728x again or a big square.