You can make the circle by taking a square of the fabric, folding into fourths, tie a string or ribbon to a pencil, pin it into the fold point with the ribbon half the desired distance, (i.e. 21 1/4" for 42 " width giving a 1/4 " seam allowance around the outer edge) and draw and arc with the ribbon/pencil compass. If you did want to make one larger you could piece 2 pieces of 42 inch fabric and have a middle seam.
It looks like it could make a nice blanket to wrap around a little one too.