yes i do agree with this because i have seen results in my site techntricksonline.com after using the 300 x 250 i am earning more click through rate then before.
well it all depends on your keyword and also how your site is design... it does not means that same add design will work for everyone.... you need to build link and earn from multipe site specially adsense sharing site.
A while ago, I bought a website where the google ads were pre-formatted and I left it that way thinking that the previous people must now what they were doing... A couple of weeks ago, I changed one of the ads, which was 120x240, to 336x280. There was an almost immediate improvement in clicks. It could be coincidence or the time of year, but I think the change in ad size did made a difference. And I appreciate the increase in revenue for whatever reason...
as personal experience i can say that bigger (ads) is better. It seems more visible to visitors and also more payd from customers.
Bigger is usually better, and place them in spots where people are looking for something to click on.
"Well , bigger ad = bigger chances for people to see it." i must say, i dont think so, 300x250 is a good choice.
Very theme and website dependant. I do just fine with 468 x 60, though I have to agree with you - 300 x 250 is good!
in my opinion 300 x 250 is best but also it is depends on your page layout. bcz some time others banners clicks by visitors more than 300 x 250.
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Thanks for your insight and for sharing your personal experiences with adsense placement. I think you should definitely play around with ad size and placement to test what's best for your site.