Usually I sketch before I swatch, but in this case, since I worked a little backwards.
The yarn here is Swans Island DK Washable Wool Merino in Malbec. The original child's cardigan was in a hand-dyed fingering weight washable wool from the Woolen Rabbit. For the grown-up version I wanted something with similar properties, but in a slightly larger, more knit-able, scale. For that, the Swans Island was a perfect match, with the Malbec a nice feminine, but grown-up colorway.
[Full disclosure: I had ID'd this as my dream yarn a while back, and then earned access to some yarn support from Swans Island following the publication of my Breakwater Pullover].
Unless I know I want a particularly dense or flowy fabric, I generally begin swatching with the needle size indicated on the ball band. The swatches here are done on US 6/4.0 mm and US 7/4.5 mm. I learned to knit on size 7 needles, and as such, I've always had a bit of a soft-spot for them.
While the 7s resulted in a bit looser fabric, the main difference in these swatches is the scale of the cable pattern. The size 6 swatch uses the same cable pattern from the childre's cardigan, while the size 7 swatch doubles the thickness of the rib and cables.
I gently washed and blocked both swatches (just flattening and not stretching), which is extra important in super-wash yarns, which I've found have a tendency to grow. Then I pinned on various place of my unnamed dress form to see how it hangs and feels in scale to the body. Unless I'm doing a sample for a publisher, I usually keep the form at my own measurements, which is a 38-39" bust.
Scale is the key here. If Lady Heartrose was a fall/winter cardigan, I'd go with the thicker version in a heart-beat, but for a spring/summer garment, I want something more delicate, so the thinner cable it is. The size 6 fabric also just looks a bit "cleaner" to me, so size 6 swatch wins overall!
Next up - practical math and complex spreadsheets!!