Facebook sets the image dimensions already based on the original image ratio. If it's a really wide, or tall, image, It can get shrunk to fit the (generally) 4:3 proportions that Facebook uses. If the image is generally square or slightly rectangular proportions and above about 200x200px, it should look right. If its still not right with that info, let us know a page from your site with a wonky image, and we'll gladly look into it further.
Thanks,
Alex
Changing image size for FaceBook posts
