Zara consistently runs about one full US size smaller than its size labels suggest. A Zara Medium often fits like a US Small.
This is partly because Zara is a Spanish brand using EU sizing translated to letters. EU sizing is generally cut for a slimmer European frame.
Recommendation: size up by one in tops, dresses, and outerwear. Pants run closer to true size but vary by line — denim is true to size, dress pants run small.
Read the measurements in cm on each product page rather than trusting the letter. Zara publishes accurate measurements per item.
If you're between sizes, the size up is almost always the right call at Zara.