the more room for the roots the better your harvest will be. in one gallon you could go about 6 weeks in veg with comfort, 7 weeks roots are getting bigger and taking up more room,
you are at 3 going on 4, i would transplant in to the 7 gallons at about 5-6 and you will have room in the 7 gal for longer veg if you wanted to go 8 plus weeks. once you go in to flower in the 7 gals you should get a great harvest but that all depends on how long you veg and how big you want her