To start with; Surface-gravity on Mars is about 40% that of Earth--i.e. SG of Earth is about 2 1/2 times that of Mars.
Now take an average human born on Earth, being 6' tall. That man or woman is 100% of their potential height on Earth, but only 40% of their potential height on Mars, because of the lower gravity of the smaller world.

Therefore, by achieving their full potential height on Mars, the far descendants of that 6' tall human colonist would be eventually 2 1/2 times as tall, i.e. 15 feet, which is the equivalent of a 6' tall person on Earth.

People on Earth can sometimes (though rarely) grow to 7' or even 8' tall. Given that, and given also that the lighter gravity of Mars would promote faster and larger bodies, it is certainly possible that people could grow to the Martian equivalent of 7' or 8', which would be about 18' or 20' tall.

This would of course have major implications for Martian architecture...