To achieve the same width when using 5.5mm needles instead of 9mm needles and casting on 56 stitches, you will need to calculate the gauge difference between the two needle sizes and adjust the number of stitches accordingly.
First, knit a gauge swatch using the 5.5mm needles to determine how many stitches per inch you are getting. Measure the number of stitches per inch in your swatch.
Next, calculate the number of stitches needed for 1 inch of width with the 5.5mm needles by multiplying the stitches per inch from your swatch by the desired width in inches.
Finally, multiply the number of stitches needed for 1 inch by the total desired width in inches to find the total number of stitches to cast on with the 5.5mm needles.
Happy knitting!