You can double the yarn, and that would probably work.
Or you can cast on more sts for each strip and knit more rows in each color. In the original yarn, the strips are about 12" wide and you have 5 of them making it 60" wide. If you don't want one that large, you would see how many sts/inch you get and multiply that by how many inches you want each strip to be. It's a simple garter stitch, so you don't have to adjust for a stitch pattern. I might add, you may want to just double the yarn to make it go faster; that much garter stitch drives knitters to poke their eyes out with their needles!