To be quite honest, the Hearts of Oak pattern is not the speediest knit around, especially when worked on US #3 needles. The unblocked fabric is fun to look at in different lights, and its almost crunchy texture invites you to grasp a handful and squeeze-- but even so, it seemed to me that I knitted endlessly on Selva before finally reaching the point where it was time to start the waist decreases.
I had swatched the "ribbon" stitch pattern I used for the top of the skirt, and measured the swatch stretched to determine whether a zipper was necessary: I decided no. As I progressed toward the waistband, decreasing steadily in the plain purl sections at the sides, I instead started to worry that the waistband would stretch out of shape: the "ribbons" weren't quite as firm as I had hoped, and of course the reverse stockinette wasn't firm at all. That's why I decided to add the elastic to the waistband.
In the case of this pattern, the sizing was dictated by the stitch pattern: the difference between sizes is two repeats or 20 stitches (or 2 1/2 inches) at the hips. Having said that, it is easy to change the frequency and number of waist decreases to customize the fit for a larger or smaller waist in relation to hips.
Filatura di Crosa "Zara" is a great choice of yarn for this skirt; it has tight twist, lots of resilience, and seems to be fairly durable (standing up well to frogging). It also comes in an extensive palette of colors. If another yarn is substituted, I would encourage using something with similar characteristics. A quick glance at yarndex suggested these as possibilities (note that I have no personal experience with any of them): Jaeger Matchmaker Merino DK or Extrafine Merino DK; Dale of Norway Falk; Classic Elite Classic 150; Debbie Bliss Rialto DK; Louet Gems Opal; or Knit Picks Merino Style. Note that for most of these (as for Zara), the suggested needle size is larger than US #3, and the gauge thus looser than the 6 sts to the inch I got in stockinette stitch with #3s. SWATCH!