I'm surprised we're apparently skipping the trope of the girls getting angry about having their measurements taken.
Right, the Manga hasn't really touched on it too much, but he's actually been making all their underwear for quite a while (as well as things like bikinis and other swimsuits). I think they mentioned him making them at some point (EDIT: Just checked, Ch 111&112, he makes some for the princess and ends up dragged into a promise to make more for the other girls), but then just kinda swept the whole thing under the rug because the scenes are R-15 and that's not the audience the manga is aimed at. Just know that if it's just their bust and hip sizes, he already knows them by heart.
He thought of using his skill to analyze their sizes because he doesn't know any of their sizes that weren't involved in making bras or panties. So to make flattering dresses, he only needed to learn things like their height, shoulder width, and waist size, which weren't part of making their bras and panties. He ended up using Wisdom though, which lends to the idea that "Wisdom" is more like omniscience than simple wisdom.