You know the word 長い from a couple levels ago. This is like the word 大きい versus 大きさ, where 大きさ is "size" and 大きい is "big." So, 長い is "long" and 長さ is length.
If you know the reading of the word 長い from level 6 you'll know the reading for this word too. Only difference is the さ at the end, which is hiragana so it should be easy to read.
length of one's foot, length of one's leg
length of arms
The length of this bamboo is three meters.
About how long is one meeting?
The length of this banana is twenty centimeters.