As I understand it, that's almost exactly what they do, even for something as simple as a T-shirt or jeans: they invite the "star" over to the boutique, or arrange a visit to the home or film set, try on sizes until they get to the closest, and then make some adjustments to make it even closer. Pretty much all of these freebie clothes are at the very least altered by a house specialist when they're handed over.
Well then, I wish these tailors were better at fitting suits. I'm always shocked at how poorly these men's jackets can fit. Not all, but many.