As I had no way to get more of the yarn, and it was knit from many stitches from the bottom up, I decided to go for the medium size. I was concerned that it might be too small, but was more worried that I would run out of yarn. So, I cast on with my US 8 needles (5mm), and knit the rest with US 4 (3.5mm) and hoped for the best. A mere eighteen grams later, I was done. Knitting the medium size was the right choice.
Now I just have to wait for Mother's Day to give this to Mom.