Choosing the right size winter hat for your child can be a bit difficult. Understanding how your child's head grows makes your decision a lot easier Bulk Winter Gloves. From newborn to size 12 months babies have a rather small head. At this age you're looking for the smallest size hat you can buy. Look for size newborn or extra small. Then as your baby transitions to 18 -24 months clothing their head size grows very quickly to about size 4T. You'll want to look for a small to medium sized hat for this age child. Small at 18 months moving up to medium at size 4T. A lot of times a hat made with a stretch knit will come in one size small/medium and this is the size hat you'll want to buy.