I've seen these described as US 00000 or 5x0 and China size 20 but at these small sizes they always show the metric size. I don't know of any major Japanese brand that offers needles this small.
This is so useful when reading patterns from other countries, thank you!
As a curiosity, Brazil uses the metric system for needles, both crochet/knitting ones. So if you're asking for a 3, you'll get 3mm needles.