A size label is not a universal measurement of fit. Brands may use different patterns, lasts, and sizing systems, so the same number can feel longer, shorter, wider, or narrower from one model to another. Even within one brand, a running shoe, a dress shoe, and a boot may be built around different shapes.
Materials also affect how a shoe feels. A structured leather upper can feel firmer than a flexible knit, while padding changes the space around the heel and instep. Toe shape matters too: a pointed style may offer less usable room than a rounded one even when the nominal size appears similar.
Use the brand's measurement chart as a starting point, then check the product's fit notes and return terms. Try both feet while standing, wearing the socks you expect to use. Walk around to assess toe room, heel hold, and pressure across the foot. Do not assume that changing length will solve a width problem.
Keep a record of sizes and models that have worked, along with what you liked about their shape. This can make future comparisons faster, but treat it as a guide rather than a guarantee. Your best size is the one that fits the specific shoe comfortably, not necessarily the size you wore in another brand.