If I know anything, I know how to make a baby hat bigger! I have three different kid/baby hats that I love to make over and over, and Ive never made them the same size yet. Same goes for kid hats. Some hats Ive made for a 10 year old, a 3 year old and their American Girl Doll! All same needle size.
Ok heres what Id do. First stay with the same size needles. Use the gauge from the hat you already knit to figure out the new hat. Just measure an inch and count how many stitches you have. Now measure the babies head. Multiply head dimension by number of stitches per inch. As was said earlier you may want to subtract about 1/2 inch or 1/4 of stitches off to create snugness. I would do the first hat flat on long circulars (ew, but you may not have size three circs with a long circ. Hmmm, Ok after you knit the band on whatever needles, I would move it to a circ (size doesnt matter, circumfrence of hat will) then you can use the circ part to put on the babies head to see if the size is going to work or not. If you need to rip it out its just the band. And if its a pattern done in the round, you can just rip it out and start over with your new dimensions.
Ive only had to rip out once with this method. I am pretty good now at just looking at the hat and the baby and adding it all up. And sadly this method only works if you have readily access to the baby.
And by the way I love this pattern, Im gona knit one up during church this morning. Much bigger as this pattern looks like a premie hat. And unfortunately my babies have BIG heads! Thanks for the pattern.