i start and finish my autos in the same pot, lowryder #2 in 4x6 inch pots. They grow between two and three feet, so no need for a bigger pot. It is safe to transplant anytime you want, just dont take clippings cause I doubt theyll root. I will however have my kushryders in a two gallon pot, as I dont know how big they will be yet.
Best time to transplant is when the soil seperates from the container when dry.