Where do you want to place the ads? Anyway, ads in the sidebar is often 125x125. Ads in the posts are often 250x250 or 300x250.
Hey Staale, Thanks for the fast answer, i am not looking for an ads size/place I mean, an image that get poster inside the blog, thanks.
If you look around at a number of blogs, you'll see there is no standard. It's whatever you feel comfortable with. I like all my images to link to a larger version, so I use the LightBox plugin and make many of my thumbnails no more than 120px high and wrap the text around them. When you click on an image that has a larger view, it expands within the same window and the viewer can either click the close button or click outside the image area to return to the post. Jim