36*36 is fine, but if you feel better making 48*48 go for it! It will still work if the child ever attends full day child care/pre-school. The nap mats at my job are 48" long, we care for children from birth thru 5 years. Btw, both my DD's still use theirs when watching tv and such. Best of luck, and please post a picture.
I think 36" x 36" is only okay for a newborn, which will too quickly be outgrown. My perference for baby quilts is approx. 42-48" x 54-60". It won't be too big and heavy for a newborn, and will work as a crib quilt or throw at the end of a twin bed.
My 16 year old goddaughter still uses the quilt of this size that I made for her as a baby. Her mom says that she likes to cuddle up in it when she's not feeling well-she has juvenile diabetes. It is a cross stitched embroidered wholecloth quilt that I made from a kit that was given to me. It has Jack & Jill rolling down the hill with the pail of water. By now, I'm sure it's tattered and torn, but I'm so glad it's been loved for so many years.
I don't think there's any set size for baby's unless you want it specificly for the crib. I've made gifts of many sizes since the mom needs blankies for: playtime on the floor, car seat, stroller, bassinet, crib, and just for cuddling. I would make it however big you want and however it will look best. Don't obsess! Enjoy!
Do you have a size that you like to make for a baby quilt? I am making one that is planned for 36"x36". The baby will be born in late March in Michigan. I'm wondering if I should add another border to make it larger so the baby can use it at least until it is a year old. To be consistent with the design I would have to add a 6" border on four sides making it 48"x48". Is that too big? I need to have this finished by the weekend! Thanks! Joyce in MI