Length
noun
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.
足の長さ
length of one's foot, length of one's leg
たての長さ
vertical length
ウデの長さ
length of arms
この竹の長さは三メートルです。
The length of this bamboo is three meters.
一回のミーティングの長さは大体どのくらいですか？
About how long is one meeting?
このバナナの長さは、二十センチです。
The length of this banana is twenty centimeters.