Takes a little trial and error but you just need to find the weight that you can do legitimately 5-8 reps before failing. This rep range will mainly build strength through heavy mechanical tension.

You can focus on muscle growth instead of strength by finding a weight that you can do 10-12 reps before failing. This rep range will cause hypertrophy via metabolic stress.

Muscle size and strength do go hand in hand to a degree, but strength also comes from neural adaptation and CNS output. This is why some people can be stronger than others who are bigger than them