The Featured Images already get resized to fit the width of the page (792 x 300px).
If you want to change this size to something else, around line 51 in functions.php, change the values in the following line…
add_image_size( ‘post_feature_full_width’, 792, 300, true );
The two numbers in that line are the width (792) and height (300) of the Feature Image. Simply change that to whatever width you’d prefer.
After doing that, install the Regenerate Thumbnails plugin and regenerate the thumbnail images for all your previously uploaded images.
After you change the above line in functions.php, any new image you upload will automatically get cropped to the new size that you’ve specified. This plugin will simply regenerate the images that you’ve already uploaded.
