This pattern must be easy to adapt, because I successfully managed it with different yarn weight, needle size and guage, considering too, that I knit this adult-sized sweater for my 10 year old. The ease was for most part due to the top-down construction and loose fit.
I simply cast on with the sts for smallest size and winged it from there. I did do little math and worked out the number of sts I needed to have at the underarm, considering my gauge. (I do not remember the exact number, as it was mostly in my head). I also drove my daughter crazy, having her try it on several times along the way. Beyond the armholes it was all plain sailing.
The yarn is our good old acrylic. I am planning on adding a third button to the sides, to decrease gaping. Other than that, it is all done and looks good.