Studs are always 1.5 inches wide. Their depth can be 3.5, 5.5, or even 7.5 inches depending on the wall. Wall studs are made with "2 by" lumber, meaning either a 2x4 (most common), 2x6, or even in some rare cases, 2x8. However, that doesn't mean that the wood is actually 2 inches wide. Nominal vs dimensional lumber can be confusing, learn more about confusing lumber dimensions here.