Remember to K3 at the beginning and end of all rows. This creates a row of ribbing at the top and bottom of the finished neck-warmer.
Gauge is not terribly important. To make a “taller” garment just add an even number of stitches to your cast on chain. Conversely, subtract an even number of stitches for a “shorter” garment. You can also make this piece closer or looser fitting by knitting more or less than the recommended 17 inches. Keep in mind that the seed stitch pattern has quite a bit of built in stretch.