This is a subject that always interests me. I thoroughly researched the pros and cons of pediatric neuters when Gage was a wee'un. As Moto has said, early (ie before around 7-8 months) neuters do delay the closing of the growth plates in the long bones, which can cause a "leggy" dog. Dogs neutered as adults (over 2 years or something like that) tend to have wider skulls and chests. Neutering early causes itty bitty pee pees (don't tell Gage!
The reason I was so worried about all of this was because a) I was worried about joint problems caused by rapid growth spurts and b) I wanted Gage to be a short and muscular as his genetics would allow, as I knew from the time I fostered him at 5 weeks that I wanted to use him for weightpull if possible.
HOWEVER - my research turned up that while the only studies that exist on this DO say dogs neutered early may grow taller than they would were nature allowed to take it's course, the difference in height is pretty insignificant, generally only 1-2 cm. Of course, they cannot measure this for sure, what they did is neutered one third a litter at 8 weeks, another third at the standard age of 7 months, and left the final third intact. There was no observable difference between the dogs in Group I and Group II, but only in the dogs left intact well into adulthood (again, I believe it was 2 years).
Long story short, it doesn't make enough difference to really matter.
FWIW, I ended up having Gage neutered at five months despite my intentions to wait, as the little stinker broke a baby tooth off and it had to be removed right away (was interfering with the adult canine coming in). Gage IS leggy, but I believe that's just his breed (suspected mastiff in his background, possibly RR). But I can't tell for sure can I? He does have a tiny little doggy dinky, but that suits me just fine, easier to rub his tum without getting grossed out
As for behaviors, I've heard neutering early can help curb humping and marking, but...Gage was neutered before he'd even thought of such things, and he's not around intact females or any males on a regular basis, yet he marks (if we get new furniture or add anything to the yard he tries to mark in the house :roll:), humps, gets the "red rocket", the works :roll:
As I do rescue, all our pups are spayed at 8 weeks. I don't feel like having to track people down and check up on them when the dog is 6 months old, and deal with the hassle if they've not gotten around to spaying/neutering as they are contractually obligated to do. So for us, the dog is fixed before it goes home!