How many hours of light are you giving them?
As far as size goes there can be many causes for the difference. May be genetics, may be sex (males tend to be taller and less compact), but there no clear rule. Just make sure they are in 12 hours per day lighting in you want them to flower and then in about 2 to 3 weeks you should see the sex of the plant.
There is no need to rotate them, but there is no harm in doing that if you don't overdo it.
"We ought never do wrong when people are looking." - Mark Twain
If the whole world smoked a joint at the same time, there would be world peace for at least two hours. Followed by a global food shortage.