You don't need to transplant until you have a good bit of growth up top. A gallon of dirt for a foot of plant is the norm.

Just for a comparison, at roughly 9 weeks from seed (which would be ~6 weeks from an average clone) I put my foot-tall girlies into 3 gallon pots and toss them into flower... by harvest time the roots are definitely right up along the sides of the pot and starting to wrap around a bit, which would by definition make them rootbound. The schedule works out well. If you wanted to veg your plants longer, say until they were like 2 feet tall, you would want to think about using a 5 gallon pot for flowering since a plant doubles (at leaast) in size the last 8 weeks of its doomed life.