Hello Erin, thanks for your question. (n__n)
【A】 If the baby body height is more than 100 cm, yes, please simply select the 'custom size' and fill up the exact body size, and our tailor will adjust the costume size to fit he/she.
【B】 If the baby body height is shorter than 100 cm, please submit a ticket (with product link and costume size request) to our Support Center and let my teammate help you directly.