The heel heights in the range
Dior's most recognisable heeled style is the J'Adior slingback, and it is published in three heights: a flat version, a 6.5 cm heel, and a 10 cm comma heel. The related J'Adior heeled slingback sandal sits at 8.5 cm with the same comma heel profile. The comma heel — narrow at the base and curving — is the design element that links them, and it is also what distinguishes the fit: a comma heel places the weight further forward than a straight stiletto of the same height, so a 10 cm comma heel does not feel like a 10 cm straight heel underfoot. If the height is your main concern, the difference between the 6.5 cm and the 10 cm is real but it is not a straight-line comparison.
The slingback strap is fixed, and that decides your size
On the J'Adior slingback, the strap is an embroidered ribbon with a flat knot bow and it does not adjust. That single detail changes how the sizing conversation should go. With an adjustable slingback, a slightly loose strap can be tightened. With a fixed ribbon, the strap either sits on the heel or it does not — a strap that slips while you walk means the shoe is too long, and a strap that leaves a mark within a few minutes means it is too short. Because the ribbon sets the length tolerance, the useful measurement is not your usual shoe size but how the shoe sits at the heel. Dior publishes guidance that some styles fit true to size while recommending a half size up; individual wearers report both sizing down and sizing up in the same model, which is what you would expect when the strap cannot be adjusted.
Toe box, material and stretch
The pointed toe that defines several Dior pumps comes with a narrow toe box, and the material then decides how much that matters. Patent leather has almost no give, so a pointed patent style has the least tolerance for a sizing error. Calfskin and lambskin are the most forgiving and will soften and mould to the foot over wear. Technical fabric and embroidered uppers sit in between: some give in the fabric, but the embroidery and backing reduce how much they move. Across the range, the practical rule is that the narrower the toe and the stiffer the material, the more reason to take the larger of two sizes rather than the smaller.
What to ask about a specific pair
Confirm the heel height in centimetres rather than relying on the style name, since the same style is published at different heights. Confirm the marked size on the pair, and note that Dior uses Italian sizing. Ask what the upper material is, because that determines whether the shoe will soften. And if the style has a fixed strap, ask for a photograph of the shoe on a foot or with the strap positioned — on a slingback, a centimetre of strap length is the difference between a shoe you can wear all evening and one you cannot.