I made oversized receiving blankets for my DD and then for all of her pregnant friends. The commercial ones aren't very big. I found you needed about 1 1/4 yards of flannelette one blanket. Wash and dry in a hot dryer. There is quite a bit of shrinkage and the fabric always needs straightening--That's why you need the extra yardage. Then square up to whatever the width of the fabric is, and hem. These will fit most babies until they are about 12 pounds.
I even made some in lightweight cotton because my DGD would only sleep if she was swaddled--even on the hottest summer day.