If I upload a large size image inn my wordpress, the image breaks my template width. I have seen some templates which scale the large image's width to the maximum width of the post div. What CSS or code I need to change for that purpose? Thanks.
e.g. if the width of your content area is 600px:
You will probably also want to add accompanying CSS, e.g.:
That way, when the image is scaled, it will maintain its proportion; rather than merely having its width reduced, the height will also be scaled proportionately.
You don't need any CSS, just go to Settings > Media and adjust the Large image size width to that of your post container. You may need to Regenerate Thumbnails after doing that.