I am currently running a kush leaning pheno of red columbian x afghani that likes to get pretty big (over 3ft tall, 2 ft wide) in veg to perform well. Similar to purple kush but don't rule out pheno variations when comparing your pk to other pk. It only seems to stretch about half the size at most (1.5X stretch) so you'll end up with a plant about 4.5ft tall by 3ft wide. My other pheno has a 2x stretch so figure out how big you want it to end up, figure out your stretch, and go from there. There's no easy answer to this question due to pheno variations but if you get it 3ft by 2ft to start I don't think you'll end up disappointed. Just don't let your temps in your room get over the mid 80's or that could make it stretch more than you want. Pruning larfy inside buds and plucking some fan leaves will let it pull a little bit too.