resizing the dimensions of the an image field
I hope you attach an image field to the content type. In that case, you can define the image styles at admin/config/media/image-styles. It is the core continuation of the Imagecache module.

Create a new style and set the dimension for the images; then, go to admin/structure/types, click "Manage display" in "Article", and click the icon under format of image field; choose the image style you created.