Halloween Costumes - Size Questions

I was looking online at the Disney Store costumes and others. Does anyone know if heavier clothes fit under the costumes? Do the costumes tend to run large or small? Did you buy your child's regular size or a larger one? I'm having a dilemma, as they are in between sizes at this point.

Thanks!

The costume I got for Luke is a Disney costume. He will be a year old this month and generally wears 12 month clothes right now. I got him a 12-24 costume and it fits him fine, it's on the bigger side but it's not way too big or anything. I was told their costumes run big but I wasn't sure he would fit in the 6-12 month comfortably. I was a bit annoyed that they don't sell the infant costumes in their stores, that's the toughest sizing to guess at IMO. He has the dalmatian costume and it fleece, very warm. I will have to see what the weather is like on Halloween but I think he will not need heavy clothes underneath. In fact, I'd say it may be too warm for indoors with the hood & all.

HTH

My dilemma was actually because the Disney site only seemed to have 12 months (which I think will be too small). I found one that had 18 months, which made me realize they must be sold out of the larger sizes in most of the outfits (I LOVE the Dalmation, especially since you posted Luke's picture! ). I can't decide about costumes, because although I try not to dress them exactly alike (usually they 'coordinate' with the same outfit in different colors) I want to avoid one being the 'sidekick', you know Batman vs. Robin kind of thing. I know I'm nuts, but it's their first Halloween and I want it to be perfect, but now I feel like I've left it until the last minute, what with disney being sold out! I guess I'll brave the stores tomorrow!