Hand knitting provides the opportunity to customize sizing and shaping to fit your body (or that of whomever you're knitting for,) and it's possible to produce garments that really fit, but even though it's possible it's not always easy.
First, measuring a body directly is complicated:
- posture impacts the measurements, and it's difficult to get measurements of the body in the kinds of shapes and positions that you're likely to hold while wearing the garment.
- ease, or the difference between the actual measurement of your body and the actual measurement of the garment, is both subjective and a matter of preference.
For this reason, I normally recommend measuring another sweater that has a fit that you enjoy as a starting point, but there are challenges:
- measurements for different styles of sweaters can have different internal proportions: the length of the sleeve depends on the width of the shoulders, and the depth of the armhole
- most machine produced garments and conventional knitting patterns are based on typical measurements and proportions which are good as starting points but typically leave something to be desired.
While people's measurements are broadly similar, and proportional, they're not the same, so if you have slightly longer arms or shoulders that are a bit more broad or angular, the "average" might be off by an inch or two, which might be enough to care about.
I'd still recommend starting from a garment that you know fits well, and record the garment's measurements as clearly as possible, but also note modifications separately. The basic idea is lay the garment out as flat as possible and measure the garment which is less likely to move than a person. There are three or four measurements that are really critical:
- width of body at across the chest below the arms.
- width of the body at the bottom hem/edge.
- distance from the middle of the back of the neck to the cuff.
- length of the sweater from the top of the shoulder to the bottom hem.
Sleeve length is pretty stable when measured from the bottom of the sleeve (where it joins the body at the underarm) to the cuff, as this avoids the impact of shoulder shape on the sleeve. Measuring arm length from a common point, the middle back of the neck, to the cuff is also a stable way to take this measurement. You may also require additional measurement's if you want the body of the garment to have contores.
While it's true that you can deduce other measurements from the four basic measurements, there are other fit considerations that are worth noting: width of the sleeve at/above the cuff and at the shoulder; depth, height, and aperture of the collar; as well as "true" shoulder width. May of these details I've figured out empirically and iteratively for myself: it's sometimes difficult to get these measurements correctly from a model garment.