How does sizing typically range on a civilian balmoral? Does a balmoral typically fit tight like a military cover (i.e. should you order a size or two larger if you don't want a tight-fitting cap)?

My future father-in-law has decided he'd like to wear one at the wedding reception (out of doors) and asked me for advice on fit and wear. Unfortunately, I don't have any experience with them myself so I offered to ask here.