This question keeps cropping up nigh on every week so this is how I look at it although there is no hard and fast rules. How you choose to grow also determines plant numbers and pot size. I.E Screen of green(scrog) or Sea of green (sog).
I think the easiest way to work out the best pot size and plant numbers is to first look at what bulb you are using, and I'm using hps bulbs as my base and a 3ft x 3ft or 4ft x 4ft space, and whether plant numbers are a concern for you or just not practical
Work out how many ounces that bulb could and will produce if your grow practices are good. Averaging 1grm/watt as a rough guide
EG..600grms = 21oz so if using 9 plants 600/9 = 66grms/plant which is just over 2oz. A 6.5ltr pot whether in compost or coco will produce that size of plant consistently if like I said earlier its grown in a good environment. I cant see the point of using big pots if only needing a couple of ounces per plant. By the time a large pot has rooted out the plant will probably be too big and you will have had to top or supercrop to keep the height down plus its just wasted time
If using 4 plants then you would look for them to yield over 5oz/plant so then you would need at least an 11-15ltr if in compost but you would get away with 11ltr in coco. Coco tends to out yield compost by more when you get to the bigger pots. Training would mostly be used for smaller numbers to fill the canopy
You also have to take into consideration vegging times. Obviously if you need a crop off quick then its no good using big pots because your vegging times would be longer so smaller pots would be the order of the day. Smaller pots are better also when height becomes a problem
If using sea of green then you could if practical fit 100 plants in there in 1ltr pots but it should still yield the same as 4 plants seeing its the light that governs your yield to a great extent but you would have no vegging time
At the moment I have a 1000w in a 4ft x 4ft space. Ive put 16 plants in coco in there and I'm looking at around 2oz per plant so chose 6.5 ltr pots
So there's a few things that you need to look at before deciding, and then decide on what's practical for you
Hope this makes it a tad clearer