gauge is what matters.
as Ingrid has pointed out, sometimes a much smaller than expected needle size is called for--but what matters is the gauge.
some designers KNIT tightly (and they might use a size 8 (US) needle to get the same gauge i can get on a size 6--
or the opposite.. they might require a size 3 to get the same gauge as i do on a size 6!
(i have friends who use size 0 needles for socks.. but my socks, knit on size 2's have a tighter gauge/fabric!)
Needle size is always just a recommendation/suggestion.
you can use any size needles--so long as you end up with the right gauge (and the right kind of fabric!)