Welcome to Knitting Help!
I'm not sure if you want just a large sock (standard garment sock) or one that has the other common shape of Christmas socks. Dustina gave you an idea for the standard one so here's another idea or two.
My first daughter was born Dec.26th and they brought her to me in stocking that looked very much like this one.
That pattern is for a felted sock so it will come out huge. If you were to use a normal needle size it might come out the size you need.
Here's a pattern for a Mini Christmas Sock that I designed that could actually made larger. It's not made like a normal sock with heel flaps, gussets, etc so it's pretty quick. Top make it larger you'd just cast on more stitches, knit for as long as you want the top, do more increases for the toe, knit longer over the toe before shaping the bottom. If you read it this might make more sense.
Here are some Santa Hats.
Whatever you choice is I'm sure it'll be adorable and loved by the mother! Be sure to share some photos with us if you can.