This is an XL men's wool knit, measuring 23 inches across the body. Excuse the poor quality image. This is an ideal sweater to make two pairs of knee-hi socks from the body, and one mid-calf pair from the sleeves. These are simple straight tube socks. Take a calf measurment for your own custom socks, and use that as your starting point. I have found that 11" works great for both of us. I would call that a medium.
The red yarn in this blurry picture *oops* marks out where I will cut. The best and most straignt forward sock comes from the middle 11" strip. Front and back will match up as one pair. The side cuts are a little funny because of the armholes. Measuring just above the waistband, I cut these at 6" from the seam, both front and back, leaving the seam in place. I make these "flank" cuts a little wider to compensate for the narrowing along the armhole. Starting at the waist band, cut straight up to the shoulder seam, cut along the shoulder seam, and down along the inside of the armhole seam. Do this front and back. Cut off the neck band. You should have something like this. A pair of flank socks.
An easy pair of tube socks.
And a pair of sleeve socks.
Remember that the waist band and wrist bands of the sweater are going to be the calf bands on the sock, and the shoulders are going to be the toes. If you are dealing with a V-neck, then your middle socks will be shorter, either mid-calf or ankle length depending on the sweater. Cut the back strip to match the front.
Now that you have your sock material cut, fold them inside out, and start sewing the seam from the calf band, down to the toe. Start with the strips from the middle of the sweater, they are the easiest pair. I make a narrow seam, only 1/4", so that the seam is not bulky. A bulky seam could be uncomfortable in the toes of tight fitting shoes. I sew the seam twice over, with a tight stitch to prevent the seam from pulling out.
Round the toe, then try the sock on, inside out, to see if it fits. You may want to make the ankles narrower if they sag, or you may find that the calf needs to be tighter. You can always make them tighter, until you get your own measurments worked out.
When you are happy with your first pair, tackle the funky armhole socks. For this pair, remember that I had cut them at 6" from either side of the seam, because the sweater was wide enough. So I started my seam at the calf, a 1/2" from the edge to maintain an 11" calf measurment. When I got to where the armhole narrows, I returned to a 1/4" seam following the contour of the fabric, rounded the toe, and continued around back to the body seam. I tried the sock on, and found that the ankle was baggy, and took it in until I was happy with the fit.
Now for the sleeves, they usually taper down to the wrist. The wrist band will usually fit nicely just below the calf. Make a mark where the sleeve gets wider than your basic width measurment (that's 5 1/2" for me or half of 11"). If you look back to the sleeve picture, the tape measure marks out where the sleeve reaches 5 1/2" wide, and the red yarn marks out where I will make my seam. Start the seam at your marker, and round out your toe as usual. Try it on before you cut the material, depending on the sweater you may need to make an adjustment.
When you are pleased with the fit, trim off the excess fabric, turn the sock right side out, and try it on again. The seam at the toe and running along either side of the calf has never bothered me.
Narrower sweaters may not allow room for three pairs. In that case I would take the middle strip from front and back to make one pair, and the sleeves to make one other pair. Women's and children's sweaters will make great short summer socks or kid's winter socks. Don't throw out those turtlenecks either, they make great neck warmers, especially for doing barnwork where a dangling scarf might get in the way. I have tried making gloves out of sweater fabric as well. Experiment and have fun!