Small size is for single pointed needles sizes 2 - 9 (2.75 - 5.5mm)
and double pointed needles sizes 0 - 6 (2.0 - 4.0mm)

Large size is for single pointed needles sizes 10 - 15 (6.0 - 10.0mm)
and double pointed needles sizes 7 - 10.5 (4.5 - 6.5mm)