Post Image Size

I'm using the featured image to place a thumbnail on the home page and a full width image at the top of each post. However, the post image defaults to the largest version I have uploaded. How can I get the post to use a more appropriately sized version? The obvious way would be to start with an image that doesn't exceed the post image dimensions. But I'd like to have the option of placing an image in the body of a post so that a user can click through to a larger version...