You can remember the meaning of 大小 with this easy mnemonic:
A fishing harbor had a shark problem where all of a sudden sharks of all <strong>sizes</strong> swam near the harbor. It was too difficult to try to attack the <strong>big</strong> (大) ones so a fisherman decided to catch <strong>small</strong> (小) sharks before they grow big.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.