This is a BIG question we get all the time and we know it can be a little confusing.
If you look at the basic sizing chart you will see there are only 2 factors that play a role in deciding which bike is right for you.
Your Height and your inseam.
Many people only consider the height, where in fact the most important factor to consider is your inseam. (which is different from your pants inseam). Your bike inseam is the length from the underside of the groin to the bottom side of the ankle.
If you would like to know how to measure it yourself you can watch this video for easy instructions.
Being tall and heavy doesn't mean that you have to get the biggest size bike if you have shorter legs you should get a smaller bike. The size of your bike doesn't affect the strength of the bike. The bike has the same carrying capacity for all sizes.