〜さ
Turning adjectives into nouns (degree of…, -ness)machine translation
Formationい-adjective minus い + さ (高い→高さ); な-adjective minus な + さ (便利な→便利さ)
The suffix さ turns an adjective into a NOUN of degree or quality: 高い→高さ (height), 長い→長さ (length), 大きい→大きさ (size), 重い→重さ (weight), 暑い→暑さ (the heat). い-adjectives drop い and add さ; な-adjectives drop な and add さ: 便利さ (convenience), 大切さ (importance).
〜さ nouns lean toward MEASURABLE, objective qualities, so they're the standard way to ask about and state dimensions: 高さはどのくらいですか (about how tall is it?), プールの長さは25メートルです. Since it's already a noun, it can stand anywhere a noun can: as subject (暑さがきびしい), as object, or after の.
Don't mix it up with the two nominalizers you've learned: こと・の turn a whole CLAUSE into a noun (泳ぐことが好きです), while さ attaches only to an ADJECTIVE. At N3 you'll meet 〜み (楽しみ、痛み), which leans toward subjective feeling — at N4, whenever you need "height, length, size," さ is the right choice.
Examples
富士山の高さは3776メートルです。
The height of Mt. Fuji is 3,776 meters.
東京の夏の暑さにびっくりしました。
I was amazed by the summer heat in Tokyo.
箱の大きさを教えてください。
Please tell me the size of the box.