5 inches wide x as long as the shorter ball of yarn lasts
Holding both yarns together, cast on 10 sts and work in the desired non-curling pattern of your choice until you reach the desired length of the scarf, or the yarn runs out.
Good stitch patterns for this are garter stitch, seed stitch, double stitch, and moss stitch. Here's the chart for double stitch, since you might not know that stitch, or call it by a different name:
Ribbing, either 1x1 or 2x2, would also work well if stitches were increased to 12 or 14 to account for the horizontal shrinking, but keep in mind that will produce a shorter scarf with the same amount of yarn.
Notes on Yarn Selection
There are so many great ways to combine two yarns to create a unique fabric. You can choose two yarns of identical or near-identical color, but different texture; or same yarn, different colors; or different yarns and different colors. One of my favorites is to choose one varigated yarn and match the other yarn to one of its colors. Bright colors look great paired with black, and pastels look good with white. The possibilities are, dare I say, endless.