I would stop at the size that you are at. If you make it much bigger chances are that the mom will only use it as a floor blanket for the baby. By the time the baby becomes a toddler and big enough to use to it they will either A want to use something more age appropriate or B use one of the other little baby blankets that they have had forever. My son is 4. He still wouldn't use a blanket as big as the finished size. His favorite ones are still small enough for him to drag around!
Love the quilt though! Somebody is going to be a lucky baby!