Relaxed fit is the design, not the sizing
A Dior hooded sweatshirt is cut with a dropped shoulder and a relaxed body, so it is intended to sit away from the torso rather than follow it. For the Oblique relaxed-fit hooded sweatshirt, Dior's own product guidance states that the style fits large and recommends ordering a size smaller than your normal size. The distinction that matters is between a design that is meant to look loose and a garment that is simply oversized on you: with a deliberately relaxed cut, taking your usual size usually produces visible excess fabric through the body and sleeve length rather than the intended drape. If you are between sizes on a relaxed-fit Dior hoodie, the smaller of the two is generally the one that wears as intended.
Fabric: cotton fleece and what it does to fit
The hooded sweatshirts in the range are built on cotton fleece, a knitted fabric with a brushed or looped reverse. Fleece is stable in the length and generally holds shape better than a fine-gauge knit, which means a hoodie that is slightly too long will stay slightly too long rather than drawing up with wear. It is also the reason these pieces soften rather than shrink: the fleece face relaxes with washing and the body feels less crisp, but the measurements move little. That is helpful for sizing — you can rely on the measurements over the fit impression — and it means the decision is best made from the actual chest and length figures rather than from how the garment looks on a model in a photograph.
Reading Dior's size letters
Dior ready-to-wear uses French sizing, which appears as letters on casualwear and as numbers on tailored pieces. The letters map onto general European equivalents, but the mapping is not the same as a US or UK brand's, so a Dior M is not automatically the M you wear elsewhere — and on a relaxed-fit piece, the mapping matters less than the garment measurements anyway. The figure worth asking for is the flat measurement across the chest, pit to pit, doubled, together with the centre-back length. Those two numbers let you compare against a sweatshirt you already own that fits the way you want, and they remove the sizing letter from the conversation entirely.
What to measure and what to ask
Lay a sweatshirt you already like flat and measure across the chest from one underarm seam to the other — that is the number to compare, and it is half the stated chest circumference. Then measure from the seam at the back of the collar to the bottom hem for the length. Send those two figures rather than your usual size, because on a relaxed-fit hoodie they describe the fit you actually want. If you already know you like a relaxed look and the style is not described as fitting large, ask whether the cut is regular or relaxed before deciding, since the same size letter behaves differently across the two.