We also use one as a bathroom rug. Because it's made with t-shirt yarn, it's absorbent, warm and very washable!
To make this knitted rug
I used The Addictive Baby Blanket pattern from Ribbed for Your Pleasure. (The pattern does make a beautiful baby blanket.)
Cut strips of 1in t-shirt yarn. The knitting needles that I used are 1/2in diameter.
Cast on 38 stitches (if using the Anne's Decorative Edge Edit). The pattern can be done in any multiple of 6 with an added stitch on each end.
About 80 rows total from the pattern.
Depending on the size of the t-shirts, one rug takes about 8 shirts.
Finished dimensions (after being walked on) approximately 25x20inches.
I feel like the photo shows more variance in the colors between the shirts than we see in natural light.