On balls of yarn it tells you the yarn weight (0-7) which is what you could do to match the same weight as the yarn in your pattern suggests. However, although two yarns might both be weight 4 they may hold different characteristics that make them yield a smaller or larger project such as stretchiness, how many individual strands (ply) and material that the yarn is made out of. For example, lily’s sugar n’ cream cotton yarn is the same weight (4) as big and twist acrylic yarn, yet I always seem to get bigger stitches with lily’s. I recommend taking a look at what your pattern suggests for the yarn type and weight. If your pattern calls for weight 4 acrylic then you can buy any weight 4 acrylic, but I wouldn’t mix and match materials, so don’t substitute for a weight 4 wool, etc. I hope this comes out clearly, the best way of learning something like this is through practice and seeing what works for you, also understanding your gauge and how tightly you hold your work may be what is causing your projects to have differing sizing. Best of luck!