resident expert???? :lol:
Like tanks, the bigger the better. Check your council bylaws as to depth as many have laws stating anything deeper than 45cm requires fencing.
Comets like to swim and can swim fast so a long pond would be preferable to something round. If you have lots of plants filtration isn't necessary but it does help. I suggest 1.5m minimum length but assume you will want to add more goldfish at a later date so if you can go bigger then do so.
You could add the white clouds to the pond but chances are the comets would find them and eat them. Besides, they are very hard to see in a pond so I would leave them in a tank and let the comets have the pond.
What do you want the pond made from? You can buy preformed ponds, use water troughs or dig a hole for a liner or make a concrete pond. All have their pros and cons.
Make sure pond is situated somewhere that gets shade with no overhanging trees to drop leaves in it, or have roots that will make their way through the pond over time.