hello , well to be honest the more room for root growth the better realy,however i would say you dont need 5 gallons of the stuff, i grow in 3 gallon square pots and put 2 plants per pot,, i use a top quality compost,say john innes no 2/3 and feed every third water, my plants top out at about between 28 and 36 inches at harvest,after about 6/8 weeks of flowering, if you feed correctly and trim the lower stuff, you should not have a prob,the worst that will happen is they will stay short, but the strain you talk about should not grow big anyway, are you sure of the strain, i have many times brought seed and what i got was not what i was expecting, i now make my own strains, and found that i can get what i want fairly easily, i hope this helps mate, and best of luck, if i can help in any way just let us know .