No, the stride length cannot currently be set manually. Instead, complete your profile as fully as possible so that it is calculated as accurately as possible.
The watch has a diameter of 43.45 mm and a polymer case with a fluoroelastomer strap, which makes it light and flexible, but it might be a little big for a 10-year-old child's wrist - check the fit on the spot.
Your son can use basic functions such as step counter, sleep recording, alarm clock, timer, vibration alarm, date display, weather and activities such as fitness or swimming, without a smartphone as many run standalone. Advanced features such as calls, oxygen saturation, stress level or cycle log often require a smartphone with Android or iOS for full use and synchronisation.