Why prune a plant that size? Particularly, an auto? Pruning auto's during their early vegetative state is probably one of the worst things you can do to an auto because now it will miss out on 3-7 days of vegetative growth while it copes with the shock of pruning. With photoperiod strains, it's no biggie, just veggy them longer. But with autos, they are on a set time limit and if they miss out on growth, it will directly translate to a lower yield because it doesn't have the extra time to recover properly. A plant that size shouldn't need pruned anyways, just use LST'ing to open up the plant so that light can penetrate the canopy. Not to sound like I'm coming down hard on ya or anything, we all live and learn
Just saying to avoid pruning unless you simply cannot get lower budsites light via LST'ing. LST'ing is as simple as tieing a string (plastic coated wire made for plant training works best) up under the very top growing shoot, and then bending it downwards and tieing the string somewhere lower on the base stalk of the plant. It can make your plant look weird and messed up at first, but as it grows it will help all bud sites to get light and stay at equal height as the main top bud site, creating multiple large colas instead of one central cola and a bunch of little satellites.
Good luck buddy, and please only prune auto's as an absolute last resort