Though the default Ad blocks is 125 x 125 pixels, you can use it for Ads of different sizes. Only make sure all are in the same size so as not to have the Ads appearing odd. You can also set the number of Ads you want for each widget. The maximum number is 8 and the minimum is 2. I said each widget because unlike other similar widgets, you can use the Ad Square widget for unlimited times. So if you choose to, you can fill up your whole sidebar with Advertisements.
Using is is a piece of cake as you just grab the banner codes and paste it into the widget. You do not have to go through the confusing method used by other sidebar Ad widgets where you need to paste the affiliate link in one place and the image source into another.
You can also opt to have the Ads rotate on each refresh and specify the paddings between each Ad. This is rather thoughtful as some might have sidebars that are too narrow to accommodate 2 x 125 Ad blocks side by side.
The only feature missing is the ability to track the number of clicks that each Ad is getting. Hopefully this feature will be available in future updates.