for what I boulder how do u make sure u stress the muscle?
The muscle is stressed when it performs its prime function.
The reason for the biceps existence is elbow flexion; to bring the ulna and radius closer to the humerus (forearms closer to the upper arm). Anytime the elbow flexes the biceps are stressed. They are the reason for that particular motion.
Rows, curls, chins, face pulls, pullups all stimulate the bicep sufficiently. The only difference between a curl and a row is the row works the upper back muscles in conjunction to the biceps. The curl completely isolates the biceps and the row teaches your biceps and back how to properly coordinate.
You can try increasing TUT for slower reps, but I don't imagine it'd do you much good. As another poster as suggested, weight gain and arm circumference is related, as with any muscular development in hypertrophy. The rule of thumb is that you gain 1 inch to your arms every 15 pounds you gain; this is a basic guideline, but not always accurate.
Keep progressing in weight, rows, pulls, and curls and your bicep development will come. Just remember the triceps are responsible in assistance with pressing movements (press, bench press) and developing them helps add to those pec muscles everyone wants to work. Your triceps also make up 2/3rd of your upper arm.
People that stimulate the biceps and not the triceps often develop imbalance around the elbow. The imbalance usually causes your arms to remain in an almost flexed position and makes it harder to extend. You'll see people walking around with an internally rotated humerus, shoulder protraction and their forearms are not nearly as vertical with their upper arms and you can tell their training is imbalanced.