Sample code for filtering an ACF (Advanced Custom Field plugin) field on update.

filter__acf_update_value.php

<?php
function filter__acf_update_value( $value, $post_id, $field  ) {
	$post_type = get_post_type( $post_id );
	$meta = wp_get_attachment_metadata($value);

	if ( intval( $meta['width'] ) < 300 || intval( $meta['height'] ) < 300 ) {
		$value = null;
		do_action( 'simple_admin_notice', "Selected image (.../{$meta['file']}) was less than 300 x 300. Please select a different image.", 'error' );
	}

	return $value;
}
add_filter('acf/update_value/name=image', 'filter__acf_update_value', 10, 3);