As tall as you allow them to!
You put a tree in the right environment and give it the right nurturing then they can grow as tall as buildings. You've never heard of trees growing half height, or only producing half the leaves they should.
So why then do we as humans do it?
Why do we accept limitations? Why do we not reach our potential, to maximise ourselves in every department possible? Why do we aim to just ‘get by’ in life; to earn just enough money, to learn just enough skills, to read just enough books?
Why do we accept settling for less? Why aren’t we growing every day until there are no more days left. You tell me another species that settles for an existence less than what they are capable of? - NONE.
Let me ask you- how easy is it to get up each morning when you know you aren’t giving everything to improve yourself? Probably not very easy.
Now tell me- how easy is it to get up each morning when you know you have a goal in mind. When you know you’re pouring it on, desperate to get going and making your dreams become a reality.
A hell of a lot easier I think.
Just remember: there is a difference between resting to renew your reserves and resting to avoid your day.