How big will my heart enlarge?

Not sure: Your heart will grow as long as your body growths through the puberty and then will stop. Its size is roughly that of your fist. However, there are health conditions that will cause your heart to expand, but that's nothing to take loosely. It weakens your heart and predisposes you to complications. The larger the worse and it some point it will give just as a stretched gum gives when overstretched.

It doesn't normally: When adult, the heart has the same size as your fist. Certain conditions such as some forms of heart failure could make it grow.