The ones I make cover most of my palm. I just cast on 47 stitches for my skinny arms, rib straight for about 90 rows, cast off loosely, then sew the side seam up leaving a gap of about 2 inches for my thumb to go through. The idea of the rib is that the arm warmer is stretchy and conforms tightly to the shape of my arm for extra warmth. Length can be adjusted by doing more or less rows. For my neighbours whose arms are more substantial, I cast on 53 stitches (using double knitting yarn and 3.25mm needles). I've found that with a firm cast-on and loose cast-off they fit most arms.