I got a request from etsy a while back for a premie headband and booties (I crochet too) I do not remember exactly how big I made it, but I had asked her for the head circumference and went based on that.
She said it fit perfect, and I made a crochet flower about 1.5" and sewed that on the headband.
She sent me lots of pictures with baby wearing the headband and booties, she looked soo adorable.
I am sure each hospital has their own rules about what the babies can and cannot wear.
Also if you want it to be a surprise you can google "preemie growth chart" and that should give you an idea of how big they need to be