A familiar size letter is a convenient shortcut within a brand you know. It becomes less reliable when you change brands, cuts or fabrics. For an online clothing purchase, a few relevant measurements can explain more than automatically selecting the label you usually wear.
Read the seller's measuring instructions first. Some charts describe body measurements; others describe the finished garment. Those are different comparisons. If the chart gives garment dimensions, choose a similar item that fits comfortably and lay it out as instructed. If it gives body measurements, follow that method instead. Mixing the two can create a misleading impression of how much room the clothing provides.
Measure the areas that determine the particular fit. For a shirt, chest, shoulder and sleeve dimensions may matter. Trousers require a different set of checks, such as waist, rise and leg length. Do not gather every possible measurement just because a tape is available. Concentrate on the areas that have caused problems before and the details the brand uses to distinguish its sizes.
Read the cut and fabric information alongside the numbers. A loose design and a fitted design can have the same size label but intentionally different room. Described stretch can influence how the garment behaves, yet it should not be used to justify a clearly unsuitable measurement. Photographs of models can help show the intended silhouette, but their proportions are not a substitute for your comparison.
Consider the full outfit and use. A shirt worn tucked in needs a suitable length for that purpose. A jacket intended over another layer needs room for that layer. Before moving up a size, check what else changes. Solving tightness in one area by choosing a larger garment may create excessive length or width elsewhere; another cut may be a better answer.
Imagine a hypothetical office shirt whose chest measurement matches your reference shirt but whose sleeves are noticeably longer. Resolve the sleeve difference before ordering, either by finding another size that still meets the chest requirement or by choosing another model. If alteration is part of your plan, establish its practicality and cost rather than treating it as a free correction.
Read the retailer's fit-assessment and return conditions before purchase. On arrival, try the garment as permitted, keeping labels attached and avoiding changes until you decide. Check sitting, reaching and the intended layering as well as standing in front of a mirror. Save the measurements of a successful purchase so the next comparison starts with a useful reference rather than another guess.