You could use smaller needle size for smaller size or larger needle size for larger size.
NOTE: If you change needle size you may need an additional skein.

I used www.yarnhygge.com Hand Dyed SW Merino - Bulky yarn for this pair in Amber color way, but you could use any Bulky weight yarn for these Fingerless Mittens as long as you get the gauge.

Only required 1 skein for each pair.