Missing function to get the defined thumbnail size metadata and a check to see if an attachment's thumbnail is actually of that defined size.

wp_thumbnail_base_size.php

<?php
/**
 * Gets the thumbnail resize sizes. 
 *
 * These are the sizes defined by the media options page or custom added sizes, rather than the actual
 * thumbnail image sizes. Falls back to thumbnail if size does not exist. Put your size parameter first
 * through the 'post_thumbnail_size' filter if you want WordPress get_post_thumbnail behaviour.
 *
 * @size either string with thumbnail name size or array with dimensions
 * @returns array with width, height and crop parameters
 */
public function get_thumbnail_base_size( $size ) {
	global $_wp_additional_image_sizes;
		
	if ( is_array( $size ) )
		return array( 'width' => intval( $size[0] ), 'height' => intval( $size[1] ), 'crop' => false );
	
	// Custom size
	if ( isset( $_wp_additional_image_sizes[ $size ] ) )
		return $_wp_additional_image_sizes[ $size ];
	
	// Rename size
	if ( in_array( $size, array( 'post-thumbnail', 'thumb' ) ) )
		$size = 'thumbnail';
		
	// Get from media settings
	$width = get_option( $size.'_size_w' );
	$height = get_option( $size.'_size_h' );
	$crop = get_option( $size.'_crop' );
	
	// If couldn't get this
	if ( empty( $width ) && $size != 'thumbnail' )
		// Get default
		return get_thumbnail_base_size( 'thumbnail' );
	
	// Return sizes
	return array( 'width' => intval( $width ), 'height' => intval( $height ), 'crop' => !empty( $crop ) );
}

/**
 * Checks if a attachment's thumbnail is of the defined size
 *
 * @attachment_id attachement to check
 * @size size to check for
 * @returns true if correct size, false if non existant or too small
 */
protected function is_thumbnail_of_size( $attachment_id, $size ) {
			
	$meta_size = get_thumbnail_base_size( $size );
	$image_attributes = wp_get_attachment_image_src( $attachment_id, $size );
	return isset($image_attributes) && (
		// When cropping, sizes needs to exactly match
		($meta_size['crop'] && ( $image_attributes[1] == $meta_size['width'] && $image_attributes[2] == $meta_size['height'] ) ) ||
		// When not cropping, one of the sizes needs to exactly match
		(!$meta_size['crop'] && ( $image_attributes[1] == $meta_size['width'] || $image_attributes[2] == $meta_size['height'] ) )			
	);
}
?>