3 gallon pots should work great for SoG. Any larger and it will impead the ability to stuff as many pots in your grow space and hinder the method.
I found a great article below that might be of help to you.
https://www.growweedeasy.com/sea-of-green
Great Luck & Happy Growing 🌱
3 gal is a good size for an auto, with 2 gallon probably being the minimum. I have grown autos in 1 gallon before and it wasn't easy, needed watering every day, hard to judge how much nutes and how often to feed. 🌱👍
3 gallon should work fine. Heck, you could use 1 gallon pots and it would still work. Larger the pot, larger the plant (generally, total time and conditions play a roll, of course).
Autos don't work like photoperiods as in a sea of green. Not effected by light change like photos. Use photoperiods, and if using a hydro soil can go smaller then that on potter. A 3 gallon is good for a auto for the length of its time growing. You can't shorten it by changing light times. You would have 5 autos full size for your sea of green. Good luck and new strategy time. 🌲🌲👊