Tiny! They’re not really needles at all, more like thin pieces of wire. The width of an acupuncture needle is around 10x smaller than a hypodermic needle - one you are given an injection with or have your blood taken with. Hypodermic needles range from around 0.4mm - 1.8mm. Acupuncture needles range from 0.12-0.35mm, depending on the length.
There are generally three lengths of needles used in practice; 1.5cm, 2.5cm (most common), and 7.5cm (I personally call this needle the ‘butt needle’ because 99% of people will need the longer needle for points on the gluteus maximus). Bear in mind that just because a needle is 2.5cm long doesn’t mean that’s how far we will stick it in. I use 2.5cm needles for most points and I usually insert to about 1cm most of the time.
If you’re worried about the needles (who isn’t…?) speak to your practitioner about it and they will put you at ease, maybe using the very smallest ‘baby’ needles we have until you’re more relaxed.