Yeah, 3/8 12g will be a tighter weave than 7/16 12g. The term is "Aspect Ratio" or AR. It's the ratio between the ring diameter and the wire diameter, and it tells you how tight or loose a particular weave will appear, regardless of ring size. On the TRL shopping site, you'll see the AR value listed with the other ring specs. The rings in your source image look to have an AR somewhere between 4 and 5. If the lowermost belt is 1.25" to 1.5" wide, then the rings are about 3/8" to 7/16", and an AR of 4-5 puts you in the 14g - 12g range. This is just guessing based on personal experience. I'd recommend ordering a couple different sizes first and make a few test patches before putting in a large bulk order. If you want to skip that step, I'd vote for 12g 7/16".
How much coverage are you looking for? Are you going to overlap the maille in the front like the picture implies, or skip rings that would be covered? Are you continuing the rings up underneath the leather harness up top? Skipping the hidden rings will shave a few thousand off the count.
Getting the maille to hang like that is going to be a challenge. You'll need a trapezoidal panel to get the 12"-14" overlap up top and the 6"-8" gap at the bottom, and you're going to need to hide in a bunch of expansion to match the artists lines (the artist cheated :-) ). Also, if you use steel, the weight is going to make the skirt portion hug your legs. Aluminum or plastic rings have a better chance of flaring out with the jupon. How are you making the jupon?