The Craft Yarn Council of America, an industry group, has issued standards for body and knitted hat sizing. While not all yarn companies and designers use these yet, many publications, such as the Interweave, Vogue Knitting,  and Knitty.com  have adopted these standards, and you can generally rely on US and Canadian yarn companies and their patterns to conform with them.  They are invaluable for anyone who wants to design, alter an existing pattern, or chose which size pattern might be best for her or whoever she’s knitting for. 

   I have formatted the CYC numbers differently  from the original, in what I think  is a sequence that better permits the reader to see the progression of measurements for sizes, from the smallest to the largest.  Emphatically, these are body measurements, not garment measurements.

   From my own trip through industry standards, I have also included gloves and mittens, as well as sizing for felted hats.  I have also added a formula for socks based on my own patterns, Socks 101 (crew socks in two variations) and Socks 201 (fitted knee socks in several variations).

   The body measurements from CYCA do not account for ease, or the lack of it; they are simply body measurements, and we’re all built differently.   A big, casual sweater needs more ease, and a fitted one needs less.  A big, casual sweater worked in a larger size with lots of ease may need an alteration to the arms, so they match your back-of-neck-to-cuff measurement, unless you want your cuffs down to your knuckles or beyond, which has been one of the odder recent fads.  A garment worked in a thick yarn will require more ease than one worked in a fine yarn.

   Some garments are fitted exactly to match body measurements; some are designed for a body-skimming fit, with two inches or so of ease; some go for a easy fit requiring about 4 inches; a very easy fit would be about 6 inches.   I have seen as many as 12 inches or even more for outerwear such as coats and jackets.  To me such ease in a sweater that isn’t specifically designed as outerwear looks excessive.  You’ll need more ease with heavier yarns, less with finer ones.  The answer to fitting a large bust may be to experiment with short rows for more length through that area rather than to make the entire sweater larger.

   How the designer intends a garment to fit will be visible in pattern photographs, and the body type intended may well be suggested by the model chosen—generally tall, thin, and youthful, though that is changing.  No one perfectly matches these standard measurements.  Another guide will be something that most recent patterns provide, which is a schematic of the garment with actual measurements that correspond to the sizes given; subtract the body  measurement from the garment measurement to obtain the ease that the designer intends.

   A basic introduction to sweater design, with an excellent section on fitting, can be found on Wikipedia: http://en.wikipedia.org/w/index.php?title=Sweater_design&oldid=71239884   And Jenna Wilson has done a clear, helpful series of articles on sizing and alternations for Knitty.com

   The most relevant for choosing a pattern and a size is at http://www.knitty.com/ISSUEspring05/FEATspr05TBP.html

Sweaters of conventional construction, with arm length measured for set-in sleeve:

Gloves and mittens, manufacturer standards:  Subtract an inch for knit gloves and mittens

Hats, knitted with some stretch, such as watch caps:

 Sizes for felted hats (such as the HCK cowboy hat)

Socks, universal sock recipe for two types of simple socks (Socks 101 and Socks 201)

Crew socks with Dutch heel and round toe:

Cast-on:  (Ankle measurement + 1 ) x (personal gauge in chosen yarn), and work 2 x 2 ribbing to desired length

Leg length:  same as length of foot

Heel flap:  An odd number slightly less than half the total stitches, to a number of rows equivalent to (heel flap stitches –2)

Gussets:  decrease every second row until you are back to your original number of stitches

Length of foot before toe decreases:  (length of foot) – (length of big toe)

Fitted knee socks with Dutch heel and round toe:

Cast on:  (measurement of calf just below the knee + 1) x personal gauge in chosen yarn, and work in ribbing for 4 inches, more if you want to turn the cuff down or secure it under knickers

Decreases: beginning with 4^th round after ribbing, every six rows until the number of stitches = (ankle measurement +1) x (personal gauge in chosen yarn).

Length of leg: measurement from middle of ankle bone to just below knee

Heel flap: an odd number slightly less than half the total stitches, to a number of rows equivalent to (heel flap stitches –2)

Gussets:  decrease every second round until you are back to your original number of ankle stitches.

Length of foot before toe decreases: (length of foot) – (length of big toe)