IME, anyything less than 32oz will stunt them.. I use 44oz. As well, be sure to use two cups.. one inside the other. The one on the inside has the bottom cut off - this reduces the amount of stress you'll cause. Before you transplant, squeeze around the bottom of the cup to loosen the roots up some. Squeeze gently, turn the cup, squeeze gently. When you first begin to remove the cup, do it while the cup is being held in a horizontal position - this reduces the amount of weight pulling on the roots and keeps the system together a little better.
Some other things I do for a great transplant are watering the pot the plant is going into with a myco inoculation like Root Drench, Pirahana, Voodoo Juice, Microbe Brew.. any myco inocculator - as well water VERY VERY little at the top of the plant you're transplanting. Stoner lore says this makes the plant believe it's about to begin drinking and charge up the roots - drop the tier cup into it's new home and cover it SNUGLY with the soil from it's new home.. water around it just to get your soil to adjust better around the cup, put new soil in place if it soaked/dropped a bit - you want it to be snug as so the roots never shift or pull when you're moving the pot or watering it.