All depends on area and plant type. I use 3 gal pots which I found to be fine for a medium to small plant indoors. They can be packed together and my plants do well in them. Hope this helps.
indoor i find 5 gals works best with space issues, MAYBE even 8 or 10 gal if i had a bigger room, outside i wouldnt fuck with pots, go right in the ground, dig a big hole and then stack a ton of dirt up so its got a nice large big motherfuckin bed, bigger the better, good luck