Field to select registered image sizes within the WordPress dashboard.
How to Use
Once activated, this plugin will create a Image Size field type in ACF. This field type will list all registered field types as a drop down within the field group.
To use this field within your custom templates you will use the following code:
$my_image_id = get_field('my_image_id'); // ACF Image return type set to ID for this demo
$my_image_size = get_field('my_image_size'); // The selected image size (eg. large)
echo wp_get_attachment_image( $my_image_id, $my_image_size ); // Output the image based on the ID, and the Image Size selected.