I can say with 100% certainty that spaying will not change the dogs body type. Slayer is just as lanky as she has always been and was spayed at 5 months. Also, if it makes you feel any better there is nothing to be scared of. Just make sure you read all the papers they give you and keep her from getting over hyper for a week or so.
Psh, I'd love it if Piper could gain weight easier! Definitely not, and she was spayed at 6 months (before her first heat). You're going to want to wait til after she's done with her heat to spay her btw; most vets charge more to spay if they're in heat.