Unfortunately, the only way to know if a set of buds fits is to try them. You can start with a guess at your ideal ear-tip size based on your physical build, but you may still not end up with your initial expectation. And then to further complicate matters, sizing isn’t standard across brands. You may be a medium ear tip for one brand, but an extra small with another. When you are trying out earbuds, they should fit snugly enough to form a seal. (An informal way to test this: Speak while they’re in, and see if you hear your voice through bone conductance, i.e., your skull, rather than the normal way.) The tips also should feel comfortable—no unpleasant stretching of your ear canal. The lack of a good seal will impact sound quality; the most highly reviewed earbuds will sound mediocre (or worse) without one. The lack of a comfortable fit will make you regret your purchase. If you’re striking out on one or both fronts, you can try third-party tips (like from Comply)—but you also may need to consider a different pair of earbuds altogether.