If you knit this with two strands of worsted weight yarn held together, that will increase the size. The best way to judge how much the increase will be is to knit a swatch over the 12 stitch pattern repeat. Cast on enough sts for 2 or 3 repeats and see how much your gauge differs from the given gauge (~5.5sts/inch).
Since the pattern repeat is 12 sts for the cable and purl sts, you can increase in groups of 12 until you have the size you want.
Row 1: (k1, p1) 5 times, (k6, p6) 12 times, (k1, p1) 5 times
Lovely soft cables on this blanket.