What matters if you want to duplicate a pattern is gauge. Gauge refers to the number of stitches per inch and the number of rows per inch. Each pattern is designed to come out the same every time if you crochet to the same gauge every time.
There are three components of gauge: 1. The yarn or thread used. 2. The size of the shaft of the crochet hook used. 3. The individual tension of the crocheter.
As a result of there being three components, you may have to adjust any one of them to get the correct gauge. Usually, you want to use the yarn or thread called for in the pattern, because that's what will provide the "look" and "texture" and "weight" of the example project. Usually, once a crocheter is experienced and competent, there's not much that can be done about tension--some people crochet loose, some tight, some medium. What can you change if you need to? The size of the crochet hook. A larger hook will result in fewer stitches per inch and a smaller hook will result in more stitches per inch (if the other two components remain the same).
Not long ago, a friend and I sat side by side using the same yarn, the same hook size, and the same pattern for a baby blanket. Hers came out much smaller than mine, because she crochets tightly and I crochet loosely. We probably should have changed hook sizes (me--smaller; her--larger); then our finished blankets would have been closer to the same size.