The number printed on a label might offer an answer, but it is incomplete. For example, a single label might appear on two shirts with completely different fits at the shoulder, across the chest, or with pants. Instead of asking “Does this fit me?” in a styling context, consider this: does this fit my outfit?
Consider how the fit of clothing interacts with the body to create a shape or silhouette. The shoulder seam of a jacket or shirt, the waistband on pants or a skirt, the neckline, and the hem on anything reaching hip, ankle, or knee. Fit influences the shape first, even before we consider color and style. A garment can be well-fabricated, have the right color, but still be incorrect if the length cuts off the outfit inappropriately.
That is why the experience of shopping and trying on clothes to ensure they fit or are not too loose often makes purchasing confusing. You might put on a garment that fits perfectly but is impossible to pair with your most favorite pants because it is too long. You may have a pair of jeans with a waist you can button but is difficult to pair with your favorite shoes since the hem sits so heavily. You might try a jacket and it closes easily but is too loose at the shoulder making the overall outfit less balanced.
To check the fit of a piece and determine if it looks good, put on a complete outfit and look in a full-length mirror. Does the neckline sit well with a layer underneath? Are the shoulders natural? Is there a definite waistline or does the fabric create a shape? Does the hem hit your favorite shoes appropriately? Move, sit, and raise your arms. Clothing that only looks good when you are standing perfectly still is not going to make your wardrobe work.
You can also use fit as an exercise. Find two sizes or two slightly different styles and put on the first with an outfit photo. Then swap to the second size/style using the same pants, shoes, and jewelry. Compare how each piece impacts the proportions of the whole outfit. Look at the photos without making an emotional decision to find which piece makes the outfit clear. Which one drapes the fabric better? Which one works with your shoes?
Beginners sometimes find it difficult because they view a larger size as the opposite of a smaller size. This makes the sizing process an emotional exercise. In reality, we wear what works for our outfit. There are times you may prefer a larger fit, you might consider different tailoring, different cuts, or you simply leave it at the dressing room because the shape does not suit your outfit formula.
When you are unsure whether you are happy with a piece, do not focus on the size first. Pay attention to the shoulders, the waist, the hem, how it moves, and the overall effect of pairing it with your own clothes. Proper fit will give you more outfit choices; the number on the tag is not going to be able to do that for you.