I have a bit of an unconventional question :D How stop stretch/grow and start flowering in autoflowers?

Unfortunately you cannot change them into flowering via photoperiod methods. I am currently dealing with 2 phenotypes of NL x Big Bud Auto Ryder, where one flowered around week 5, and is small and bushy - the other is around 2 feet taller and began flowering 2 weeks later. All on the same 24 light period - same tent, same nutes, etc. So with auto's sometimes it is a challenge in that regard! You can expect them to double or more in width and height when they begin flowering.