Costume sizing rarely matches everyday clothing sizes, which is why so many Halloween outfits arrive too tight across the shoulders or long enough to trip over. A few measurements solve almost all of it.
Measure, do not guess
Take chest, waist, and height with a soft tape over light clothing. Age-based labels are a rough guide only, and children of the same age can differ by two sizes.
Compare against the size chart, not the label
Always check the specific chart on the product page. If your child falls between two sizes, size up. A slightly loose costume can be belted or pinned; a tight one cannot be fixed.
Leave room for layers
Halloween evenings are cold. Assume a long-sleeved top and leggings underneath and make sure the costume still fits comfortably over them.
Check length and sleeves
Hems that touch the ground are a tripping risk on steps and kerbs. Sleeves that cover the hands make carrying a treat bag awkward. Both are easy to shorten with a quick stitch or a fold and safety pin.
Order earlier than feels necessary
Aim to have the costume at home two to three weeks before Halloween. That leaves time to try it on, fix a fit problem, or exchange it without paying for rushed shipping.
Do a full dress rehearsal
Costume, layers, shoes, and accessories together, for at least fifteen minutes. Anything that itches, slips, or catches will show up right away.