You can remember the meaning of 小时 with this easy mnemonic:
Have you ever noticed how small (小) each day is, compared to indefinite time (时)? You may think one hour is long, but think about the millions and billions of hours that's already existed over the past few centuries! One hour doesn't seem that long now, does it?
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.