Quantity of rice per diameter of paella pan?

I don't think you'll find the information you are looking for, for multiple reasons:

• The most direct one: because the same pan can serve well a really wide range of rations. We've cooked excellent paellas with the same pan from 4 up to 30+. (more or less 80cm pan)

• The most practical reason: You usually WANT to cook too much paella, because the leftovers put on a fridge for a day or more and then re-heated on a microwave are tastier than the dish from the same day. You actually look for leaving half the dish or more. So quantity is usually the opposite of a problem: It was either soo fine that everyone took a second serving and cleared the pan, or it could be a little better, which it will be the next day. You literally can't go wrong, as long as there's a lot!

• Lastly, but not less important: It really is not a dish prone to measurings, up to the point that the amount of rice is measured in fistfuls, and it really does not matter that much. The result depends too much on the quality of the batch of ingredients used, the meat and/or seafood, the quality of the broth, how well kept the wood-fire was, the proper judgement of salt point during cooking and if you timed each step appropriately. As long as the pan is big and flat enough, it will no longer be a deciding factor in the quality of the result.

If I was deciding upon sizes, I'd use the references on amount of servings provided by the manufacturer, but only as a guideline, and tend towards picking larger than smaller.

Edit: One serving, at least where I'm from, is two handfuls. If your hand is little, add another handful at the end to round up. Usually we prepare servings for 50%-100% more than people are there, for the sake of being able to serve a second round and/or letting some on the fridge for eating the next day. At home, this ends up being 17 handfuls for 4 people, 7 servings approx, on a paella pan more or less 80cm across.

Per serving, we also add what we call "tall" (translation would be 'cut', stands for the meat and seafood, excludes vegetables): a pair of prawns, a pair of mussels, two or three "Clams" (https://en.wikipedia.org/wiki/Venerupis_philippinarum), a "Galera" (https://en.wikipedia.org/wiki/Squilla_mantis), a cuttlefish cut in dices and rabbit. The cut that needs to be cooked for longer than the rice itself is added to the same paella and cooked there before adding the rice and the broth or water in it, so the juice of it all stays in the dish.

Source: I've been eating paella cooked by my family my whole life, who are born and raised among the Delta de l'Ebre rice fields. And stringently judging among themselves each sunday paella (for 70 years!). Among the criticisms, the amount of servings for a paella has never been even mentioned. I'll ask the cooks at my next chance, but sadly I live abroad now, so it can be a while. I should make a video of the process, and upload it if it ends up a 8 or more.

A free tip: If it sticks to the bottom of the pan and that bottom is slightly crispier/darker than the top rice, as long as it's not actually burned, it's fine. In fact, it's the best part of the dish.

As @rumtscho says in his comment, it should scale quadratically. Your 40 cm diameter paella pan has an area of pi*(d/2)^2 = 1256.64 cm^2 (lets round that to 1250), while your 26 cm pan has an area of 530.929 cm^2 (lets round that to 530), so less than half the area. Consequently, the amount of rice (and stock) compared to your large paella pan will be less than half. For the sake of simplicity, I would half the amount, then round down to the nearest convenient unit or tens digits if it's a really odd number (like, say, 233 ml of something).

The more exact ratio is 1250/530 = 2.358, or approximately 2.4, if you want to "divide down" your amounts for the 40 cm pan to the 26.

Note: all this assumes you can get similar heat per area and the two pans heat similarly. Experimentation may be required.