I always try to pot-up twice, not including what the seed starts in. If you must go from small to large w/o an intermediate step, water carefully around the plant for a while, increasing the circumference a bit each time, don't do a normal soil soak. You know how big the root mass is, initially anyway. A pretty good rule of thumb for plants in general is that the roots extend out to the far end of the branches/leaves, aka the 'dripline'. Concentrate your watering efforts there until it's big enough for regular watering.