Today was the day to solve the loom size question. Mission accomplished.
First Greg and I sat down to work out the basics of how the loom should look and operate. That took some time because not all the period depictions look alike. We had to figure out a working set of assumptions based on our experience with building and weaving on upright looms. I'm pretty confident in the design decisions he made; after all, he's done this for me many times now.
Then I had to talk my patron out of expecting a piece three meters wide. I didn't confront the problem straight on, but took a more circuitous route. For one thing, I pointed out how wide all the looms were in the period depictions. Even the widest are not much larger than one person's "wingspan," as Greg put it. If there were ever a case of letting the correct tool lead us to the correct product, this will be one. But because I let the tool's innate logic speak for itself, I never had to hit him over the head (metaphorically speaking, of course!) in order for him to come around. Eventually, after he interacted with a physical sample of cloth in the correct size range, there was a very satisfactory meeting of the minds. So now he is commissioning a piece that is much closer to correct, Greg doesn't have to make a cloth beam so long it's structurally unsound, and I don't have to weave half again more cloth than I expected. It's all good.
Last night I acquired some more technical specs for extant textiles that include thread counts. I am really enjoying how the research process keeps turning up exciting facts that upend old assumptions.