For instance if I have a custom size, and I want the admin to be able to select that size, or I want the return url to be that size no matter what, that would really helpful.
I might not be understanding correctly, but I’m not sure why you aren’t just doing this as part of your template file. That seems to be where this type of thing would belong.. You set up your custom image sizes, customer uploads a photo, WordPress makes all the respective sizes, and then you simply call the right size in your template file.
<?php $theimg = get_field('grid_image'); ?>
<img src="<?php echo $theimg['sizes']['customsizehere']; ?>" alt="<?php echo $theimg['alt'] ?>" />
If I’m missing something, can you clarify a bit more?
