Following that lengthy monologue with a bit of housekeeping: I should let you know that a few Ravellers have noted that the Elfbaby hat comes up big, usually when working the Pumuckl or Pippi borders.
I'm pretty confident in my sizing overall – I compared various sources to get the best, most definitive measurements and I made a lot of samples myself, besides having it test knit, in various sizes and border options. But the Pixi border pulls in a lot more than the others, which evidently makes a big difference to the fit of the whole hat. So I'm recommending that you pay attention to gauge, and consider going down a size if using one of the other border patterns. Remember too that the design is intended to accommodate growth, in that the border can be turned up (tightening the brim as well as shortening the hat) for smaller babies and turned down as they grow.