Halloween Costume Sizing?

Do they normally run small or big? This is the first year we are dressing up our dd. She is 15 months and is wearing size 18m. She can still wear some 12m clothes( carters onesies and certain pants) its just the length that is a lil short... My question is should I get a costume that is size 12-18m , or 18m-2T? The costume that I am thinking of getting comes with a separate piece for the head so I am thinking to possibly order 12-18m ? I don't want it to be too big and flopping all over. I don't know what to do!! Of course I wait til the last minute as usual!! TIA !!

Halloween Costume Sizing?

I'm sure it depends on the brand and specific costume, but I would go with the 12-18 size. DD is in 18 month clothes (she'll be 18 months tomorrow) and fits into some 24m stuff as well. I got her an 18 month elephant costume and she's got plenty of room in it but it fits well. I had also gotten a snow white costume in case the elephant didn't go over well and it was too long and she was tripping over it meanwhile the sleeves were to tight on her arms. Thankfully she loved the elephant costume bc I had to return snow white.