Size
noun
In Japanese, さ on the end of a word is often the equivalent of "-ness" in English. In other words, if you replace the い on the end of an い-adjective with さ, you have a noun. In this case, the adjective is 大きい (big) and the noun 大きさ means "bigness", aka size.
When a vocab has okurigana (hiragana attached to the kanji) it is probably going to be the kun'yomi reading. Since you learned the on'yomi reading with the kanji 大, we'll have to use a mnemonic to remember this word.
When you saw the size of (pick something surprising), you said "おお*#$@!" Really imagine something surprising. Say the おお part out loud, as well. Think about the size of whatever you're looking at, too, and how big it is.
ベッドの大きさ
the size of the bed
スーツケースの大きさ
the size of the suitcase
アメリカの大きさ
the size of the USA
このサンダルの大きさは、28センチです。
These sandals are size 28 cm.
アメリカでビックリしたこと？それはドリンクの大きさですよ。
What surprised me about the United States? The size of the drinks.
このチェストフリーザーは二人くらい入る大きさですよ。
This chest freezer is big enough for two people or so.