Before you can modify your image sizes in your child theme, you must first locate them in the files so you know the exact image slug to target. The add_image_size() functions are located in two files:

Avada -> wp-content/themes/Avada/includes/class-avada-init.php

Avada Core -> wp-content/plugins/fusion-core/includes/class-fusion-portfolio.php

To easily locate this, just do a text search in the file for add_image_size.

Once you’ve found the image size function, you can copy out the image size slug that you’re wanting to modify, for use in your child theme’s function.

Note: The “portfolio-five” image size is used for not only the five column portfolio layout, but is also used in the Recent Posts element’s “Thumbnail on Side” layout.

Avada -> wp-content/themes/Avada/includes/class-avada-init.php

Avada Core -> wp-content/plugins/fusion-core/includes/class-fusion-portfolio.php

Now that you have the slug for the image size you would like to change, it’s time to move to your child theme.

Step 1 – Go into your child theme folder and open the functions.php file. If you don’t have this file, simply create one with the same filename.

Step 2 – At the very bottom, add this action:

Step 3 – Replace the value ‘blog-large’ inside remove_image_size( ‘blog-large’ ); with the image slug of your desired image size.

Step 4 – In add_image_size( ‘blog-large’, width, height, true ); , replace the values with the exact same image slug and add your desired width and height values respectively. For example,

Step 5 – If you’d like to change more than one image size, copy these actions and paste them inside the same my_child_theme_image_size() function and change the necessary values accordingly:

Step 6 – If needed, run the Force Regenerate Thumbnails plugin to regenerate your thumbnails.