Keep in mind that the size of the romaine lettuce head/hearts varies, so use your best judgment on how much dressing you need to use.
Toss the lettuce with the dressing right before serving. If you let it sit too long, the lettuce will wilt and lose its crunch.
Use a large bowl to comfortably toss the salad before serving. If you use a small bowl, you will end up bruising the romaine salad leaves which can ruin your salad and make it soggy.