You wont be able to determine their sex untill theyre a week or so into flowering, give or take a few days. There is no set height for veg or flower. You can start their flower cycle when theyre as little as 8? tall and have atleast four sets of leaves (or so ive read). Or, you can wait till theyre 4? tall. It all depends on how much you expect to harvest and the amount of room u have for them to grow. Since they double, or more, in height during flowering if you only had four feet total height fpr them to grow in youd want to start flowering when their about 20? tall, a little less than half the space you have available. If you had 8? clearance you could wait till theyre 3?+ to flower. That part is up to you and your space restrictions. Now, of course, the smaller the plant come harvest the less product youll get. How much space in height do you have total? Sorry that this post is jumbled but im at work, on my cell, now...