If you want to grow the absolute smallest plant possible, “12/12 from clone” may be a better choice than 12/12 from seed. A clone is a piece of a bigger plant that was removed and forced to grow roots. Since a clone is technically the same “age” as its mother plant, it’s pretty much a mini mature plant as opposed to a seedling.
Because they’re already mature, clones start flowering almost immediately after getting 12/12. That can result in some REALLY small plants at harvest. Unlike seedlings, clones will start flowering almost immediately after getting 12/12. If you start with small clones in miniature pots, you end up with short buds on sticks at harvest. Though not necessarily the most productive when it comes to actual yields. Auto-Flowering Strains – When Time is of the Essence. Some growers want to use 12/12 from seed to get to harvest as soon as possible, but it’s not actually the best option for a quick harvest. If you’d like to be able to harvest plants even sooner than 3 months, consider giving autoflowering plants a try.
Most autoflowering strains are ready to harvest just 2-3 months from germination. That’s less time than almost any photoperiod plant will take, no matter what light schedule you provide. Even better, autoflowering plants on average yield 1-2 ounces each (more if you care for them well), which is better yields than most “12/12 from seed” plants. The “modern” version of 12/12 from seed is simply using an autoflowering strain. They don’t need special light schedules and are ready to harvest in just 2-3 months from germination. Autoflowering plants are typically given 18-24 hours of light a day their whole life, which allows them to get bigger in less time than plants under a 12/12 schedule. Auto-flowering strains are ready in 2-3 months from seed, and yield an average of 1-2 ounces/plant. You can yield even more with low stress training (LST) and great care. Use “Sea of Green” (SoG) to Maximize Photoperiod Yields. If you want to grow photoperiod strains (or can’t get your hands on autoflowering seeds) but still want to produce big yields as fast as possible, the “Sea of Green” grow style may be a good option. Sea of Green is simply the idea of growing many small plants instead of just a few bigger plants. It works great for photoperiod strains to produce a quick harvest and great yields. It’s kind of halfway between “12/12 from seed” and a typical grow. For this grow style, growers usually switch to 12/12 when plants are 4-6 weeks old. Since adding a little extra time to the vegetative stage often increases yields, you need to find a balance between getting to harvest as quickly as possible versus harvesting the biggest yields. For SoG, wait to switch to 12/12 until plants are about this size or a little bigger ( note: young plants like this can and will double or triple in height after the switch to 12/12) SoG in Action – Make sure to fill the entire space under the grow light with plants and you will get a sea of buds! I think an autoflower is a much better choice for that. My 12/12 from seed was stretchy, produced 20g per plant. They weren't that small (they were tall for their proportions.). Autoflowers would have been as tall (or shorter), but more fully formed. The downside to autos is that people typically run 18/6 through flower. That's 50% more energy and wear/tear on your light. Even at 12/12, I'm sure it would yield more than a photosensitive 12/12 from seed. Update: I see from your 2nd post that you're trying to get an early harvest (time-, not space-constraint which I responded to).
Autos are best suited for outdoor growing to get an early harvest. I think that and micro-growing indoors (implying no outdoor space, or wrong time of year) are the "valid" uses of autoflowers. (I put valid in quotes because many people just like to grow them. But, if you're already growing outdoors, an auto would be perfect. (A little longer than 12/12 from seed, I suppose.). Fundamental response to "have you ever thought of" questions: We've been growing weed, lots of guys, for 40 years personally, and other guys longer than me. Methods that differ from the common methods, have undoubtedly been tried, and, if they worked, would have been adopted and become known and standard, if they worked better.
If your goal is to do well, it's best to learn to rock the standard method before deciding to experiment too aggressively, IMO.