More important than how large the dressing room is, is to define how many meters of wardrobe or shelving I need, and from that the dressing room is determined. And, I have already seen floor plan examples here, where the dressing room was 8 sqm or more and you could barely place 350 cm of wardrobe usefully. For me, I also define it more as a walk-in closet rather than a dressing room. Dressing room in the sense that I also dress there requires more space between rows of wardrobes than a walk-in closet, from which I only want to take clothes or put them back.
I planned the wardrobes themselves exactly with the Pax planner... so storage planning. And I ended up with 300 cm of open wardrobe and 60 cm of open shelving for one person. But everything is accommodated there, including summer or winter jackets, trousers, etc. And as I said, I never wanted folded T-shirts again, so everything except sweaters is hung up for me.