How big is a newborns head?
it depends on the newborn.
some people (and some babies) have small heads--for a newborn, this might be 14 inches round..
some people (and some babies have HUGE Heads-my son had the nurses gasping.. he had a 20 inch head!(some adult don't have heads that big!)
Fortunately knitting stretches!
a 16 inch hat is a good place to start for a newborn.. it might be a bit big (for a week or two!) but babies grow fast!
The easiest (and best fitting!) is a simple ribbed hat. (knit flat or in the round) swatch to find out how many stitches you need for 16 inches (in stocking knit) --correct the number (you want a number that is a multiple of 4) GOING UP in stitch count to get correct number)
(add 2 more if knitting flat)
work K2, P2, (k2 -an extra half rib if knitting flat)
for 6 to to 7 inches.
decrease (first change to K2, P1 pattern) then to a K1, P1 pattern then to stocking knit (all with P2tog, or K2tog)
then another round or 2 of k2tog.. (until you have 10 or fewer stitches)
drawsting them closed, (and if needed, sew seam)
a ribbed hat will fit a newborns head, and will still fit 3 or 4 months latter!
(the Sweet baby hat--free on ravelry is one hat pattern that comes in size 0 (newborn) 6 month and 12 month size..
its a fun hat to knit, (a single ball (50gm) of sock yarn will do it.. and most sock yarns are superwash--so the hat will be washable.
self striping sock yarn makes a pretty hat.