At what week or size should you start Scrog? and a...

8-12" is the standard height to start a scrog so it depends upon when they hit that height (probably at the end of the 3rd week). You should do all of your topping first although scrog doesn't require you to top. Most people top at the 3rd node, I personally prefer the 4th although I have no reason but gut feeling for that. I've seen people wait until their plant is 2 ft tall and then carefully bend down their plant to 12" with a scrog net. Seems likely to break the main stem or tear at the roots so I'm not ballsy enough to try that. Hopefully this helps!