Noun
ながさ
おかしいなー、ちゃんと長さをはかったつもりなんだけどなー。
Weird, I was sure I measured the length properly.
こちらのヘビは3センチたんいの長さで切り売りはん売しています。
This snake is sold in three centimeter long pieces.
僕がよろめいてしまった主な原因は橋の長さです。僕にはちょっと長すぎました。
The length of the bridge was one of the main reasons why I faltered. It was just too long for me.
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.