I feel your pain, I spent all day on this, but I got it to work. I posted separately about it, maybe this will help you?
http://support.advancedcustomfields.com/forums/topic/custom-image-size-with-image-field/
I ended up having to use “Image ID” instead of URL, but the code posted there is exactly what I’m using.
The only change would be to choose either true or false, for the image size code in the functions file, here:
add_image_size( 'homepage-thumb', 97, 83, true );
I have mine set to true so it doesn’t distort the image – check out this page in the WP docs, about “Crop Mode” :
http://codex.wordpress.org/Function_Reference/add_image_size#Crop_Mode
Hope that helps!
