36" square is about the usual size for a baby blanket, but you can make a smaller one that can be used for tucking around the baby in a car seat or stroller.
A very easy one is this, knit in garter on the diagonal:
CO 5 sts, knit 2 rows then -
k3, yo, knit to end of row. Repeat this row until the size you want.
To decrease - k2, k2tog, yo, k2 tog, knit to end of row. Repeat that row until you're down to 5 stitches, then BO.