Strength is about neuron reactions from the brain to the muscles. You must wire your brain to perform quick bursts of strength by training it with low rep lifting, in turn it will force your muscles to adapt and gain size after your muscles have been rewired to lift heavy. Once more, the more muscles involved in a single rep, the more your whole body is being required to perform the task. This being said, when size and strength is the goal, multi muscle exercises are the key because it makes your body release growth hormones in only way that multi muscle exercises do. High rep or single muscle exercises don't make you release growth hormone at such a high rate as the former.
Note: Low/high rep is referring to how many reps you can possibly do before not being able to perform any more. Not how many you want to do before deciding to put them down.
Start at weights where you hit failure at 6 reps. Keep using those weights till you can accomplish 12 reps in all exercises. Move on to higher weights once again where you hit failure at 6, repeat.
I know what you're thinking. "Why shouldn't I just start off with something I can do 12 reps with?" Well, you will never push yourself to be as strong as you could be. Sure, someone who never does anything lower than a 6 rep range will be stronger than if they're never picked up a weight in their life, but they will only be half as strong as they have the potential to be.
It's true that when first you start lifting with 6 reps you won't gain muscle mass as fast you would with 12 reps, but after a few workouts you'll soon be able to lift 7, then 8, 9, 10, 11, till you're finally able to hit 12 with weight, and guess what? You'll be twice as strong as you would be if you were working with you're initial 12 rep ability. Now, what about muscle size? By the time you're able to hit 12 with your new weight you'll now be working in a twelve rep weight to gain that muscle mass but you'll be doing it with weights twice as big! Now keep on lifting with this weight or up your weight to something you can only do 6 reps with again and work your way up to another 12! Some more great news is that if you do all this while using multiple muscle exercises you are making your body release a crapload of growth hormones which further up your muscle mass.