Well if I were You. I might just dig a good sized hole about twice the size of a 5 gallon pot and fill it with as much of your soil as possible and put it in there, But if the only reason you want a pot is to bring it in and out because of Weather you would want what Majorduke Suggested 5 gallon would work just fine, Good luck Northern Lights is one of my Favorites Deep Pungent Skunky Smelling dank Mother.
5 yes would be sufficient. ...however for future auto grows...u should plant her and keep her in the pot that you intend on flowering her in. Autos have a difficult time with shock due to transplanting.