it depends on how wide the scarf is, how thick your yarn, how bulky the st pattern...i would gather the scarf up in my hand and see how large a circle you needed to keep it from slipping and then take the diameter of that circle....d=c/3....so 9 inch circle? 3 inch diameter...
I just made a very easy keyhole scarf. There were 15 stitches in the pattern. After 5 stitches I cast off 5 stitches and the continued in the pattern. On the next row, on the way back, I cast on 5 stitches at the same place I had cast off 5 and continued. The resulting hole was large enough for the end of the scarf to pass through. This was very easy and it worked.
I use this easy method. Divide the stitches in your row in half. Knit half of the stitches, place a marker and then attach another ball of yarn and knit the remaining stitches with the new ball of yarn. Continue knitting your rows this way (knitting with the correct ball of yarn on each side of the marker) until you have the number of rows that will make an opening to allow your scarf to pull through. Then rejoin the row by starting with the side that is being worked with the original ball of yarn; knit to the marker; remove marker; break off the second yarn ball and continue to knit the rest of the row and remaining rows using only the original ball of yarn.
The great thing about this method is that it allows you to adjust the length of your keyhole to get the size that you need.
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