These are a great project to use up the small bits of yarn that accumulate in any knitter's stash. I will use whatever I have, be it acrylic, superwash wool, or even my own handspun, but not mixing types. Making these large in non-superwash wool and then felting them to fit makes for a sturdy slipper.
Size is dependent on the yarn used, and you'll be making these often enough that you'll work out for yourself the appropriate needed needle size for the yarn for your most comfortable knitting. As a general guide, bulky yarn and size 9 needles will make a men's, Worsted weight and size 6 needles will make a woman's size, and fingering yarn and size 2 needles will make a child's size.
This project is a good beginner project, as the very basics of knitting - Cast On, Knit, Purl, Increase, Decrease, and Bind Off - are used.
CAST ONCO 6
STARTING TABKnit 6 rows in garter st (knit every row)
Continuing in garter stitch, Inc 1 stitch at each edge until you have 19 stitches on your needles.
INCREASE ROWK1, YO to last stitch, K1 (you should have 36 stitches)
BODYKnit 40 rows in Stockinette stitch (knit 1 row, purl 1 row
DECREASE ROWKnit 2 together across row - 18 stitches
ENDING TABIn garter stitch, Dec 1 stitch each edge until you are back to 6 stitches
Knit 6 rows
BIND OFFBind off the 6 stitches
FINISHINGFold your rather funky-looking piece of knitting right sides together and the beginning and ending tabs together. Using the tails from your cast on & bind off, sew the beginning 6 stitches to the last 6 stitches. Match the ends of the increase and decrease rows, and sew up the sides of the body.
A SHORT DISCUSSION OF GAUGESince I'm not making a specific yarn recommendation and these slippers stretch to fit a wide variety of feet, I'm not being very specific on gauge for this project.
Gauge can be defined as the number of stitches and rows per inch for the specific combination of knitter, needle size, stitch used, and yarn. It can vary quite a bit.
The swatches in the following photos all have 23 stitches and 23 rows. One swatch of each pair is in stockinette stitch, and the other is in mistake stitch rib.
The swatches in the above image have not been blocked. They show how mis-shaped pieces of knitting right off the needles can be.
This pair of swatches are done in a rather slubby "art yarn". The yarn might not be the best choice for slippers unless the slippers were made large and then was firmly felted.
These swatches also show that some yarns are less than effective at showing the details of a stitch pattern. This is something to think of in projects.