This sleeve should fit a 13" or 15" laptop
29" Inch Circular Knitting Needles, Size 8/5mm
Each Column .22"
Each Row .17"
Cast on 115 stitches.
Starting at row 1, knit in the round all the way to stitch 115. You'll now be at row 2, stitch 1 in the pattern even though it's actually a spiral.
My changing colors needs some practice, but this should give you an idea of what it looks like.