Adsense is much like popular culture and this is especially the case when discussing size. Does size matter? Some argue that it is not the size but how you use it. The similarities between this popular culture argument and adsense on blogs is incredible. Like real life, the question when posed with regard to adsense is impossible to answer definitively. Does size matter when using adsense on blogs? Yes, but how you use it is just as important as size. To state that the bigger the add the better the click-through-rate (CTR) is ridiculous. The success of an adsense block is very dependant on ad placement in conjunction with sizing. Big adsense blocks may have great success but can cause ‘ad blindness’. On the other hand, cleverly placed and sized ad blocks may have better success on certain blogs as blending ads in makes the reader not consider the ad block as an ad but rather as an extension of the content. Adsense offers a wide array of colour and sizing options, you should play around with the options and choose whatever will blend in and not be ad-like on your blog.
I think the size of the ad actually can matter. Of course, other factors such as the site design, content, and placement on the page will affect what size ad works best.