Just pick a size. You can grow Autos in 1 gallons, (and stunt them a little sometimes) you might get a half or even an ounce or two per plant. Ide choose anything 5 gallons and above, especially for outdoors, if you want high yield. Some can get quite large. I grow them indoors In 10 and 15 gallons sometimes. Yields are always over 100 grams(for me) in large containers, but I've had some 10 and 14 ounce monsters. I have a 3 foot tall untopped Ethos Pineapple Runtz going now in a 3 gallon. The plant I'll be using in my Tropical Runtz line breedings