Suggestions like this should be submitted here https://www.advancedcustomfields.com/contact/, I know, the title of the forum is Feature Requests… long story.
As for a solution that can be used now. Generally when I want other that the full size image I get the image ID and then use wp_get_attachment_image_src();
$image_id = get_field('image_field_name', false, false);
$image = wp_get_attachment_image_src($image_id, 'size i want');
echo $image[0];