"Three gallons beath the proper size for the growing of thee finest and stoniest cannabis, so sayeth the Lord" (I think it says that in the Bible or the Torah or the Koran or maybe the Declaration of Independence)
We're talkin indoor gro rite??
One gallon is too small - at first it seems okay, but it is a total bummer when you realize you have a killer plant in a tiny lil' one gal pot. A gazillion one gallon pots are alot of work too! Hard to keep nutes balanced. They are hard to water correctly when the plant gets big too. PLUS, high quality seeds cast what $5-$10 each? Do u really want to use a one galllon for a $8 sed? If you have the time and energy (or if you just gotta keep 'em small) a bunch of one gallon pots tend to yeild a bunch of nice dense buds.
I like three gallon office wastebaskets from Staples or Office depot (drill a few holes in the bottom). THEY ARE BEAUTIFULLY SQUARISH (9"x12" - 20% more efficient than round pots for capturing light). 3 gallon pots seem to keep indica and indica-sativa hybrids just the right size to create the right size canopy beneath indoor lighting. By this, I mean to say that (with skillful topping or super-cropping) most of the buds tend to form within effective illuminance of a 600watt HID. By this I mean to say, that if you do the math, the effective illuminance range of a 600 watt HID is (about) 9'-15" from the bulb: closer than that and the risk of burning is too high, farther than 15" from the filament reduces the light density to below 3000 lux.
Hypothetically, I suppose that a 5 or 7 gallon pot for one single sativa might be okay, especially for vert lights on light movers, but I don't have any experience with this method.
For outdoor?
Well that depends on how big the plant can get before some do-gooder snitch dickweed reports it to the poli or scams you plant.