I totally understand that.nothing worse than cutting a feature many people use. but i only ask myself one question. what image size is wordpress actually using if you select e.g. thumbnail even if it is unset in the functions.php. e.g. the preview size was set by default to thumbnail on my side and the image size used looked much much bigger than the thumbnail size i would have expected. i just rechecked it. if you set the preview size to thumbnail and it is unset acf and wordpress use the full size instead. same procedure if you use medium or large. always the full version is used instead of the unset one.
