This poncho is for standard gauge knitting machines with patterning ability. It is really just two pieces of  knitting. Stitch changes create the flare. 
SIZE, GAUGE, YARN & MACHINE  Sizes finish to a circumference of 34,36,  38, 40, 42,44,46,48 around the narrowest  section which covers from neck to shoulder  area. Circumference around the bottom edge,  which falls in the hip area is 53, 56,59,62,  65, 68, 71, 74”. Circumference around the middle section which encircles the bust, arms  and waist is 45, 48, 50,53,55,58,61,63.5”. Total length is 13, 16, 19, 22, 25, 28, 31, 34”  These correspond to girls size 7 to ladies size 16. Use Heirloom 3/11 or Tamm Sport 7  yarn to get the gauges below. Country Knitting of Maine carries Tamm Sport 7. Using  stitch size 10 on the Studio 328, the gauges, with generous use of weights are 5.3  stitches/13.3 rows/inch in the tuck pattern, 6.2 stitches/7.5 rows/inch in stockinette  and 8.2 stitches/15 rows/inch in slip stitch. Passap machines can make most sizes and  mid-gauge machines may be able to match the gauge and make the small sizes. 
As a rule of thumb, make ponchos 16-20” larger around in the middle section than that  actual bust measurement to allow for arms and freedom of movement. Larger sizes require more extra room than smaller sizes because the arms of adults are bigger and the  movements are larger as well. Extra room does not cause a problem but less feels restric-  tive. In case you are unable to measure the intended wearer, industry standards for girls  state that sizes 7-16 have bust measurements of 25.5, 26, 28,30, 32,34. Ladies 6-16  bust measurements are 34, 35.5, 36.5, 38, 39.5, 42. These figures are averaged from  catalog data. You will find significant variation from brand to brand.