Hard to train an auto unless you know what your doing.Training is easy,Getting it to grow fast enough to train before it goes into flower is a whole nother story.your auto will fit in a 2x3 no problem.training it to take up the entire 2x3,that should be a challenge.You would have to have a good seed also.not all auto seeds are the same.so out of a 6 pack you might get one that would do it but your still back to providing a stellar environment for it to reach that potential.topping and training is really for photos where you can veg them out for a longer period to fill in your space.You would be better off training 2 or 3 plants in that space to get it filled in.If only having one plant is your only option than just let it grow and bend the top over when it gets to tall.It will lay over but start growing back up.just pinch it and lay it over.It will form a knot and stay that way.Some guys get all crazy and finger fuck there plants but it aint that serious.just pinch and bend.theyll come around.