This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of short? It's short.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
What's a very short race? A midget car (みじか) race. Don't tell me you've never heard of midget car racing.
Isn't that skirt a bit too short?
The baby's short, flailing arms and legs are so cute!
Wah! I didn’t know your tail was shorter than mine.