So I'm not exactly sure when Google Adsense made this new feature available. But I learned of it this past week that you can now make custom advertisement sizes. The option is located in the size list, just above the Link Ads group. I did test this out with 320x180 on my site. What I've found was that image ads are resized to fit the largest portion of the ad. So an image ad that's normally 300x250 would show up 216x179. It allows you to use different sizes, but your not guaranteed its going to fill the complete space your after. The text ads work well with it as they adjust better than an image designed for a specific size. Anyone else here test this new custom size option out? Any luck with it?
For image ads, (Most of the time) it get re-sized and shows smaller ads. The space around it remains unfilled. It tends to work in better manner for Text ads. Advertisers usually select ad sizes so there's a chance so i will prefer to stick to standard IAB banners. This way i know how my ad will look and no surprises.