Hi! We make this size in sxab already. 😊 For sxaa/sxma18964, there are no plans to expand to this size as a stock product at this time.
There is currently 1 special of this size on the sxaa18 page, in the sxaa18532 section, in ice.
This size can be ordered as a custom size. Contact firstname.lastname@example.org (at) gmail.com is you need more info.
Any chance you will consider making 18 9/64 in BA? Sure other suppliers have them but not in all your beautiful colors. Using square 3/16 for JPL is not always an option. And that is not the only great weave that requires a AR of 3.
I'm new to maille myself, but I'll throw out some of the concepts that have occurred to me when considering your post.
It's hard to tell from the image, but that appears to be "Dragon Tooth" or possibly one of the "Half Persian" unbalanced variants with chevron base sheets joined in three orientations to create the 3D effect. The most visible rings look to be in the realm of 1/4"-ish. The bits at the top of each triad of chevrons that tend to flop out could be corrected with the creative use of slightly smaller rings to eliminate the slack. Overall, I would doubt that the maille in the image is 1/4" thick and should be pretty flexible.
As for an adjustable size, that has traditionally been resolved by the incorporation of a series of buckling straps horizontally attached in a vertical line on each side from the bottom edge up to a few inches below the armpit. The straps can either bridge a slit, compress the sheet to reduce the overall diameter, or cause the sheet to form an S fold with the extra surplus maille. Part of the decision of how to accomplish the adjustability of the finished piece will be dictated by the pattern and it's orientation. Many patterns have a "grain" direction in which they don't want to compress which may be dramatically different from the behavior when compressed or stretched in some other direction.
The choice for small rings is where you can quickly get into some trouble. Smaller rings can really look fantastic and the addition of colored rings means that the artisan can create a much more detailed shape or image or pattern with the color, but the number of rings increases incredibly rapidly. At a minimum, using a ring 1/2 the size will require at least 4 times as many rings. Likely much more for more interesting and complex patterns.
The numbers are more intuitive in metric than imperial, so we will use those in this example. Lets say that you chose a pattern that ideally requires an AR of 5.0. You could use a ring that is 25mm across made of 5mm thick wire. (That's about an inch across with wire about as thick as a stack of 3 US Quarters). That is a seriously chunky appearance and very thick sheet. (It might be better to use those monsters for ring maille where the rings are attached flat to a cloth or other flexible material.) If you choose those nice, sexy smaller diameter ring of only 5mm made from 1mm thick wire, you will get the same stiffness or flexibility, and exactly the same pattern appearance, though with a smaller pattern unit size, but it would require at least 25 rings to cover the same area of the pattern as 1 of the large rings.
Using our 10 seconds/ring assembly speed, the large 25mm ring takes 10 seconds to add. In order to create the same area of maille with the 5mm rings, you would work 25 rings for a total of 250 seconds. (4 minutes and 10 seconds) That is simply the calculation based on area. because maille requires that each ring overlaps with at least one other ring, the actual time for the 5mm rings would likely be closer to 5 minutes for a simple pattern, but could be much, much longer, depending on the complexity of the pattern and how many overlaps there are with each added ring. The reason is that the number of overlaps and depth of each overlap will reduce the area that each single ring adds to the surface of the workpiece. It's always a tradeoff between production time and ring size.
Hope this helps!