The mittens in my Grýla Collection are one size only but pattern says that more sizes can be achieved by changing needle size. It is indeed an effective sizing method. Here is an illustrated example of it. The 3 Rósir mittens were knitted with different size needles by different knitters: the blue mittens were knitted by knitter A on needle 2,5 mm / US 1,5 and makes for a size small. The red mittens were knitted by same knitter A with needle 2,75 mm / US 2. The purple mittens were however knitted by two knitters: both are of the exact same size but were worked by knitter A with needle 3 mm / US 2,5 while knitter B obtained the same gauge using needle 2,5 mm / US 1,5. Knitter B would most likely have to go down a size 2 mm / US 0 in order to make a small size like blue mittens.
So make sure to check you gauge well and don’t hesitate to play with needle sizes!