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.
The size of this table is 100cm in width and 120cm in length.
The size of the TV is 32 inches.
Ronald McDonald always wore different-sized shoes.