All of my garment patterns have six sizes, covering from XS (28-32 inch bust) to 2X (48-52 inch bust)
I tend to knit very loosely, and I have decided to list my needle size as the standard in my self-published patterns. You will know you are a loose knitter if your stitches slide easily on your needles, the loops of yarn are visibly larger than the needle, and you find it easy to k3tog.
If you are a tight knitter (your stitches are tightly wrapped around your needles) you may want to start your swatching with a needle 2 or 3 sizes larger than I have listed.
In my patterns, and any other pattern you use, if you want your finished project to be the size in the photo/schematic, you MUST make a gauge swatch and adjust your needle size.
Something as simple as getting 6 stitches to the inch instead of 5 can result in a sweater 10 inches too small!
In addition, I heartily recommend that you check your gauge periodically as you knit. Stress can make you work a bit tighter. Your level of comfort with a stitch pattern and the speed at which you work can loosen your stitches up.
My patterns give the gauge per 1 inch - to convert to rows/sts per 4 inches(10 cm), simply multiply the gauge by 4. So 4 sts to 1 inch is the same as 16 stitches to 4 inches. And 6 stitches to 1 inch is the same as 24 stitches over 4 inches.
When you make your swatch, make it at least 4 inches wide/long and measure your gauge over 4 inches.
If you are using more than one color, use all the colors in your swatch, and wash it in warm water to check if the colors bleed onto each other.
Check the gauge of your swatch before you wash it. Wash and dry your swatch as you plan to care for the project when it is complete, be that handwashing and a stiff block, or machine wash and a round in the dryer. Once the swatch is dry, play with it a bit and check your gauge again. Some yarns will be about the same, others can grow or shrink significantly. Don't get me started about the "superwash" yarns that felt when you wash them, or the red and blue yarns that bleed the first 20 times you wash them. Make a swatch, wash it, and be warned, or suffer the consequences when the first wash of your beloved item changes it beyond repair.
When I size my patterns up, I do so paying particular attention to the fit in the larger sizes.
Body and sleeve length is the same in all my sizes. Height corresponds roughly to girth IF all those included are roughly the same BMI. But look out the window at those walking by - height does not correspond with girth in the real world! The length I use in my patterns is about right for those around 5'6". My main concern was that those who are wearing the 2X don't end up with the headache of figuring out how to shorten the sleeves and body by 6-8 inches, as can happen with conventional sizing methods.
Yoke depth increases somewhat over the sizes, to accommodate "deeper" shoulders and larger arms.
Shoulder width increases somewhat over the sizes, but that increase slows down in the upper half significantly.
While I appreciate how elegant an un-shaped garment looks on a slim figure, I don't love how it looks on the other end of the size spectrum. In light of that, most of my yokes have different shaping for each size to give a more tailored fit on the upper end. This prevents problems like shoulder seams slumping over towards the elbow and ultra-deep armholes.
The take home for all of this is that if you've been burned by the average xl or 2x pattern with too long sleeves, too wide of shoulders, and weird yoke fits, you might want to give one of my patterns a shot. If you'd like to give one of my patterns a shot in the larger sizes, send me an email and I'll get one to you free.
My Sizing Standards
Most of my lace patterns are charted and written out line by line.
Many (but not all) of my colorwork patterns have written instructions as well as charts (all the colorwork is charted, but not all is written out)
Large sections of stockinette with just a small amount of shaping (like in a sweater body) is written out only.