Casting on 30 stitches with worsted weight yarn (e.g., Caron’s Simply Soft Solids) using US size 5 circular needles fits my iPhone 5 (in its case) perfectly. Casting on 36 stitches will work for the smaller iPhone 7. I didn’t have any iPhone 6s willing to model for me.

You can really make up any color scheme or pattern. The main value in what I’m posting is the method and specs.