Locate Current Image Sizes
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
Fusion 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.