Sounds like you are still in veg bud & in that case they are just going to keep on growing so you need to think about total height of the plant before flipping to 12/12 with HPS where average stretch is normally double the size most of the time i don't think i've grown a strain yet which has tripled in size.
Mmm ye it is best to check strain info as well as some times height is mentioned perhaps small, medium & tall some seed breeders mention this in centimeters etc kind of gives a little idea about things...
It is really the total size of vegged plant what matters which will over all govern total size of end plant once stretch has finished in the first few weeks of flowering.
Mostly i'll grow short & medium height strains for tent growing on average some where between 12 to 18 inch tall when flipped with final height some where between 24 to 40 inch in height once done but final height is from several different strains grown tho from seed.
You can take clones from these plants if you wish to keep the strain going a little longer if you like it & i myself would do this just before flipping to 12/12 taking the lowest limbs for clones they don't do much in the first couple of weeks as their trying to sprout roots but ye grow pretty much like any thing else with out issue, may be a slight less growth vigour than a plant from seed.
After all some growers keep mother plants which clones are taken from lots & lots those clones all form good plants.
In less some thing is wrong with your environment which may cause excessive stretch ?
Well ya been around here long enough to know about light distance etc
But ye their is some stuff about ideal temps which may help ! -
Control your cannabis - Tips on height control
Never seen that post by the way in till today, but ye i run temps around those figures it works... some thing worth pondering over
