if you have close spacing as in a sea of green then about 14" tall is good maybe 12" if you have tight node spacing and like 6 nodes or more in 12" you will be fine for that type of growing.

You will need lots of air movement with close spacing.

Watch your Ph and please change your water & nutrients every 7-10 day's. If you want post some pictures and description of your grow equipment and your grow area and maybe my self or some others can give you ideas to improve your success.