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 sizes swam near the harbor. It was too difficult to try to attack the big (大) ones so a fisherman decided to catch small (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.