Answering this here, so as not to clutter the breaking news thread:
Moderna tested (at least) 3 doses in mice, which differed by factors of 10. The middle dose looked good enough, so they tested that (100µg) as well as 25µg and 250µg in humans. You can see some of the results in this video. (I’ve edited, and probably replaced all the “mg” with “µg”, but originally said: Um, I don’t know how to print a “mu” in this software. Read all my “mg” as micrograms, not milligrams.)
The stuff about doses starts just after 30 minutes in, with mouse data, and some results of the human trials can be seen starting at 34:38. They found 100µg better than 10µg. At 40 minutes, you can see that 100µg worked better than 25µg, (especially for neutralizing antibodies, and also for persistence of antibodies) but 250µg looked about the same as 100µg. So they went with 100µg.