Following a pattern for a crocheted garment can be tricky because gauge is tricky in crochet. You might match the stitch gauge but not the row gauge. You might want the fabric to be looser or more firm than what you are getting by following the pattern. You might very well be using a different yarn than the pattern indicates. There are lots of factors.
So, knowing what kinds of measurements are involved can be really useful. And it turns out that there are size charts out there that include a lot more information than just the bust/chest measurement that we usually depend on when buying ready-made garments. The Craft Yarn Council has pages on their website with this information. Here are some links:
The kinds of information shown on this page and listed in the body charts
give you a clue of what kinds of measurements to look for, for example, with making doll clothes, too.
These charts are handy tools in my information kit.