The heart pretty much grows with the rest of your body, so it is about as big as your fist - that is true both when you are a tiny baby and also when you get older.
So basically, when you stop growing, so does your heart.
I found a similar question asked on Blurtit actually, which you may want to take a look at here.
As well as information on its growth rate, I also found some fun scientific facts about the heart which you might not know:
- Before you are even born your heart beats about 60 million times.
- The human heart will beat about 2.5 billion times in an average lifetime.
- Mary Shelley, author of Frankenstein, kept the heart of Percy Shelley, her dead husband, wrapped in silk until her own death.
- The human heart is capable of generating enough pressure to squirt blood up to 30 feet!
- The heart of a blue whale weighs about a ton and you could actually crawl through it's aorta.