Ummm WHICH birthday???? wow thats gonna take a while....As much use as a body pillow gets, I dont think I would knit it.
I think i would sew it out of material. If it was just a normal pillow, MAYBE, but this is a body pillow...wow...I knit/crochet couch pillows all the time but never a pillow case for a bed pillow, much less a body pillow.
If you want to knit it, I think i would go with the biggest yarn you can find and the biggest needle you can find, get the measurements of the pillow and just garter stitch it. I once made a blanket done on US19's with 3 bulky weight yarns held together....It turned out pretty.
Good Luck to you......