The number in the name of a silk square is its size
Louis Vuitton names its silk squares after their dimensions and the product pages confirm them: the Square 90 carries "90 x 90 cm" and is printed on pure silk. The house's own category copy then contrasts the sizes directly, describing the 70 cm square as giving a lighter drape and the 55 cm square as a compact accent. That is the whole sizing question on a square, because a silk scarf has no fit: what changes with the size is how much fabric you have to knot, how much of the neck and shoulder it covers, and whether the printed motif reads at a distance. A 90 cm square folds to a triangle that sits on a shoulder; a 55 cm square ties close to the neck like a bandeau.
Shawls are quoted two ways, about a centimetre apart
The large shawl is the accessory most often described by its size in a conversation, and the published figures are close but not identical. The Monogram Classic shawl is listed at 143.4 × 143.4 cm — a silk and wool blend with a fringed hem — while another shawl reference on the same site reads 142.5 × 142.5 cm. That is a centimetre of spread between listings rather than a change in the product, and it is the reason a shawl measured at home can sit slightly under the figure you were quoted. The men's Monogram Classic scarf is a different object again: 185 × 39 cm in pure wool, a long rectangle that is wrapped rather than folded into a triangle.
Caps and hats are sized by head circumference
Louis Vuitton caps are listed with a letter and a circumference at the same time, and the circumference is the figure that decides the fit: the size M line reads 58 cm and the size L line 60 cm on the leather and cotton caps, and the trucker cap prints the same 58 cm for M. Hats that are not adjustable are described by their outer dimensions instead — the fleece bob is listed as 30 × 16 × 30 cm — which tells you the depth of the crown and the width of the brim rather than what will fit your head. Measure your head where the band will sit, in centimetres, and compare that with the M and L lines; converting from a letter in another brand's cap is the step that usually goes wrong.
Sunglasses carry a lens size and a filter category
Two Louis Vuitton frames that look similar on a face can carry quite different lenses, and the product page states the size inside the lens: the metal round frame is marked 53 mm and the metal pilot 63 mm, a centimetre apart across the widest part of the lens. The same pages give a sun lens filter category — category 3 on the Waimea L — which describes how much light the lens cuts rather than how wide it is. When you ask about a frame, quote the lens size marking together with the frame name: the size tells you how the sunglasses sit on the face, and the frame name tells you which pair you mean.