The whole neuter/spay or keep intact issue is very debatable
There are good, solid studies to back up every view-point, so the best advice is to talk at length with someone you trust... generally your vet, or your dog's breeder.
IMO, spay/neuter before physical maturity will affect the dog's growth, but if their genes provide for decent structure they should be fine - you'd never even notice for a pet or low-level competition dog. I think they should mature fairly similar to how they look at the age you spay/neuter.
If you are planning on agility at the national/world level, then the age of spay/neuter would mean a lot more... but females are essentially always spayed since a dog in heat isn't allowed on the show grounds, and males usually are neutered so they are less distracted or wanting to mark.
At the service dog program, temperament is much more important than structure (as long as the dog can work comfortably), so all males are neutered by around 5-6 months. The females are spayed before their first heat cycle, unless they are being considered for breeding. Fortunately, in the lines they work with, many of the females don't have their first season until 18-20 months of age - by then they are mature enough that it's a moot point.
I second the weight gain! My mom's Toy Poodle wasn't spayed until age 6, and she gained a little weight that we've never been able to take off (in 6 years!!). Granted, she's only 8 lbs now, but her build has changed... no more sleak figure - even though it's mostly muscle, she looks like a tank!
But don't let that scare you off, lol! Younger dogs rarely show any signs of change after surgery, and are usually back to themselves within a couple of days. When they say it will calm a dog down, that doesn't mean they stop having energy, but rather that they are more content to relax and hang-out until *you* are ready to go - then they can go-go-go until you're ready to stop
