The preview size is only what is show when an image is selected in the admin. You can use any size of the image on the front end for the most part, with the exception of returning the url for the image.
There are tree return values, Image Array, Image URL and Image ID.
If you set the field to return the image URL then the URL returned will be the full size image URL, so you’re locked into this size.
If you return an Image Array then all of the information about the image and all of it’s sizes are returned.
If you return the Image ID then you can use that ID to get any image size you want using standard WP function like wp_get_attachment_image_src() http://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src
All of these return values and how to use them are covered in the documentation for the image field. http://www.advancedcustomfields.com/resources/image/
