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.
おおきさ
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.
このテーブルの大きさは、よこ100センチ、たて120センチです。
The size of this table is 100cm in width and 120cm in length.
テレビの大きさは三十二インチです。
The size of the TV is 32 inches.
ロナルド・マクドナルドはいつも大きさのちがうくつをはいていた。
Ronald McDonald always wore different-sized shoes.