i live on the island of Mykonos and can see Naxos from my home.  Donoussa is on the other side of Naxos from me but I've visited it a couple of times a few years ago.  To say it is small is an understatement and hardly any of the locals speak any English but that is one of the appeals of the place.  Ferrys though are hit and miss, running like the UK on GMT but in this case GMT means Greek Maybe Time - maybe the ferry will turn up on the day it is due, maybe it won't!!  It is not an island to go to if you are on a tight schedule and you need to arrange to leave a few days before your flight home to give the ferry time to turn up.
The beach at Kendros located about a kilometer from the  island's capital Stavros, is a swimwear optional beach, but is actually a nude beach. It is regarded by many people as one of the best naturist beaches of Greece.  It is the largest beach of the island with very green sea water.  There is a small bar two minutes away from the beach which gets quite crowded by naturists on hot days.  The bar is not naturist !  A little bit further away the beach Livadi is largely a naturist beach as well. Probably there are still more small beaches on Donoussa outside the villages where you can discretely swim without swimwear but like many of the isolated places, they are difficult to get to.
I would recommend the island for a couple of days, but unless you are really into isolated places with little nightlife except for the local taverna 2 nights is enough.  Hope this helps.

Thanks Peter

Very helpful.

How about Antiparos then or a twin centre between the two?

I have never visited Anti Paros although I have a friend who has a villa there.  I understand that there is only one nudist beach on the island (the main island of Paros has more.)  Getting to Anti Paros is fairly easy with ferrys running from Paros quite regularly, however, like many of the other smaller Greek Islands, Paros does not have an international airport so its either a flight to Athens and then an internal flight (very expensive) or a charter flight to Mykonos and then take the ferry.  A nice cruise of a couple of hours.  The nudist beach on Anti is called Soros and is a few kilometres north of Chora, after the small Village beach, and it is supposedly a paradise beach for nudists. Official nudist beaches are rare in Greece; nudism is common, but rarely allowed. This beach however belongs to the tiny group of official nudist beaches, and consequently it is very popular, not only with people staying on the island but also with day trippers from Paros. The only downside I am told is that it attracts quite a few "textiles" who like to walk up and down ogling the genuine sun worshipers. The beach is more predominatly naturist though. At a swimming distance opposite the Nudist beach is the small island of Diplomats, where there are some small strips of beaches that are also frequented by naturists but there are no facilities on these so it is a case of swimming out there armed with your own supplies in waterproof bags if you intend staying for any length of time.  Accommodation on Anti Paros is not a problem as there are more hotel rooms than tourists.  Hope this helps.

It does Peter

Thanks