Sizing questions are one of the most common pre-purchase messages in apparel, which tempts a lot of sellers into writing an exhaustive, detailed sizing FAQ covering every possible scenario — but a shorter, sharply targeted version usually gets read (and prevents more returns) than a long one that shoppers skim past or skip entirely.
The two or three questions that actually drive most sizing confusion, across most niches, are: does this run true to size, small, or large relative to a standard size chart; is this a unisex or gendered fit, and how should someone size accordingly if they prefer a different fit than the default; and what should someone do if they're between two sizes. Answering just these three clearly and specifically covers the overwhelming majority of actual sizing hesitation, without needing to write a comprehensive garment-construction essay.
Specificity beats generality here — "this runs true to size" is less useful than "this runs true to size in the chest, but the body length is about an inch longer than a standard tee, which some customers like for a slightly relaxed, layered look." The second version actually answers the unspoken follow-up question a shopper has, rather than requiring them to message you to find out what "true to size" even means for this specific product.
For the "between two sizes" question specifically, a clear, opinionated recommendation ("if you're between sizes, we'd suggest sizing up for a more relaxed fit, or down for a snugger fit — most customers in between go with the size down") is more useful to an undecided shopper than a neutral non-answer, since it gives them something concrete to act on rather than leaving the decision entirely in their hands with no guidance.
Placement matters as much as content, same as with any FAQ — a short, specific sizing note directly in your listing description, near the top rather than buried at the bottom, prevents more messages than the same information technically available but harder to find in a separate policies page a shopper may never click into before deciding whether to buy or message you first.