Basically, I put the island pretty much equidistant from those four locations.
Interestingly, I can't see there being great surf beaches on the Island in this location, as the East China Sea is only about 100m (300') deep and the Island chain to the East will break up the Pacific rollers...
The fact that I put the city to the South East of the island was arbitrary, as with the size. The map I drew makes the island about 30 miles from end to end and about half that from side to side, but as I said that can be changed easily, as can the orientation of the island - I can make it straight, bi or gay.... Sorry, just kidding... Paintbrush is an amazing little tool in the Windows 7 version, so I can rotate the island, flip it over, stretch it, skew it, whatever anyone wants...
I confess, I know nothing of ocean depth and surfing physics xD;
In my head while writing up the location, I had it in mine that the island was just about large enough to fit a the city proper; a few mountains, a rather dense forest. In addition, there would perhaps be some farmland cleared out, around the city; keeping it as self-sufficient as possible and all. I imagine the traditional diet of the city largely consists of seafood and hunted game from the forest, but they'd at least likely try to have some rice paddies and/or vegetables to round out their diet.
Hey, I think I just answered a question that no one asked. xD What is the traditional Zan'an diet?
Anyway, I'm not so good at figuring out how much space that would require, but basically... city proper (however large that is), rural farmlands, forests and mountains. As for the shape, I had it in my head that it was somewhat round, perhaps with one of the larger mountains at the center of the island. Don't ask me about the geographical validity of a volcanic island at that location, it was simply the appearance that popped into my head.
Perhaps there's also a few smaller outlaying islands (ya know, the tiny sort that don't even appear on most maps), just a few places which pirates could hide away in little forest-covered islands.
Also, while reading the posts concerning the name I had a thought of my own, that the name of the city came from the same root as Nan'an, perhaps meaning Southern Peace as well, if the city is focused on the southern side of the island, not in any real Chinese, but rather a fictional regional dialect (which may or may not have faded out in favor of more common dialects, such as Mandarin, as mentioned in the demographics answer. "Chinese" is actually a collection of many related regional dialects. Just look at how different Mandarin and Cantonese can be.