The cut is in the product name, not in the size letter
Look at the full name of a Balenciaga garment and the fit is already there: the house lists a "Political Campaign Large Fit T-shirt", a "taped balenciaga back long sleeve medium fit t-shirt" and an "oversized t-shirt", so the same silhouette can appear under two or three different fit words in one season. That word, not the letter, is what changes the volume of the garment — a Large Fit tee and a Medium Fit tee in the same size letter are cut on different blocks. When you ask about a piece, send the fit word from the product name as well as the size. A photograph of the item laid flat beside a tape measure resolves the question faster than either word alone.
What the house itself says about sizing down
Balenciaga's own listings carry the sizing instruction, and the two lines that appear most are not identical. One reads "This garment fits large, we recommend sizing down." The other, on pieces cut deliberately loose, reads "Designed to be worn as an oversized/very loose fit, we recommend sizing down for a regular fit." The difference matters: the first line tells you the garment is bigger than its letter, while the second tells you the looseness is the design and that sizing down converts it into an ordinary fit. If you want the look the house cut the garment for, the second line is not an instruction to take your usual size — it is a warning that your usual size will read as oversized on purpose.
Where retailers put a number on it
Because the house does not say how many sizes to drop, retailers often do. Fit notes on named Balenciaga pieces read "fits large to size — we recommend trying one size smaller" on most hooded sweatshirts, and "fits large to size — we recommend trying two sizes smaller" on the large fit logo-print hoodie. Those notes also carry the model reference, which is the second half of the answer: a 178 cm model photographed in a size 0, a 185 cm model in an XS. Where two sources disagree, the flat measurement settles it — and the disagreement is worth knowing about, because it is why one shop tells you one size down and another tells you two.
Measuring a Balenciaga piece before you ask
Lay the garment flat and take three figures: across the chest from one underarm seam to the other, which is half the stated chest; from the collar seam at the back of the neck to the bottom hem for the length; and across the shoulders from seam to seam. Compare those with a garment you already own that fits the way you want rather than with your body, because on a loose cut the gap between you and the fabric is the design. Women's ready-to-wear is numbered in French sizes and the casual pieces usually carry a letter at the same time — XXS to XL — so a size conversation needs both. One practical note on care: the house recommends dry cleaning for ready-to-wear unless the label inside the garment says otherwise, which also means the fleece is not going to shrink into a neater fit.