125x125
250x250
300x300
468x60
Those are the 4 most common sizes for wordpress.
125x125 is generally in the side bar
300x300 is a side bar
250x250 is generally in the post toward the top
468x60 fits across the bottom or across the top of a post.
Also check it
http://www.fileformat.info/tip/web/imagesize.htm