One house, four size languages
Balenciaga product pages in the cited set show a denim jacket at size S, a tailored jacket at Italian 46, a zip-up at French 36 and track jackets on numbered labels. A number on one jacket is not automatically the same quantity on another. The category filter itself can offer an Italian run beside a letter run. Before converting anything, record the system printed on the label and the system used by the listing. If the listing says only three, ask whether that is a 1–5 label, an Italian 3-something, a French 36 shorthand or a letter equivalent from a retailer table.
Fit words are per style
A regular-fit denim jacket, a large-fit tailored jacket, a normal-fit single-breasted jacket, a slim-fit jacket and an oversize zip-up can all sit in the same brand. The words describe different blocks, not rank. Some oversized pieces state that they are designed for a very loose fit and suggest sizing down only if a regular result is wanted. That sentence is more useful than a retailer's blanket advice because it names the intended result. Read the fit line with the model height and size worn, then compare the garment in hand.
A numbered label is not a tape measurement
The 1–5 system on track jackets does not represent a chest circumference, back length or shoulder width. Retailer charts treat it as either a height band or a translated letter, and the two readings disagree by a full letter in the middle of the range. That makes the number a location in one maker's run rather than a spec in centimetres. Record the reference and the actual flat chest and length. A size 3 that fits one wearer at 170 cm and another at 180 cm does not prove the chart is wrong; it shows that the number is not a body measurement.
Shoulder and hem separate two similar jackets
A blouson with elastic cuffs and ribbed hem will pull the lower body inward, while a straight denim or tailored jacket allows the hem to sit open. Both can have a similar chest and shoulder width when laid flat. Ask for the hem opening and the centre-back length alongside the chest. The fit of a zip-up also depends on whether it is designed to close over another layer. Measure the jacket you already like the same way and compare the four numbers, rather than using the word oversize to predict how much extra room is present.
The jacket evidence sheet
Record shoulder seam to seam, chest underarm to underarm doubled, centre-back length, sleeve from shoulder seam, hem width and your own chest circumference. Add the fit word, the system used on the label and all model or reference information. Take front and back photographs with the jacket flat. Those six figures make a Balenciaga size question answerable even when two charts disagree, because the finished garment from the exact reference is the object being compared.