You want your image field to return “Image ID”. With the ID we can apply a custom size to the returned image.
$attachment_id = get_field('field_name');
$size = "full"; // (thumbnail, medium, large, full or custom size)
$image = wp_get_attachment_image_src( $attachment_id, $size );
// url = $image[0];
// width = $image[1];
// height = $image[2];
?>
<img class="image-class" alt="" src="<?php echo $image[0]; ?>" />
<?php
More info about this in the documentation at http://www.advancedcustomfields.com/resources/field-types/image/
