I worked at Levi’s Corp for 3 years and was surprised to learn that clothes have large tolerances in sizes to keep costs down.
True fit says you should select a size based on the size you wear in another brand. That won’t work because no brands have consistent sizing. Even if a brand used extremely tight size tolerances, brands actively size up and down their clothes based on the whims of merchants.
The solution is to just buy 3 sizes of each item you want and select the best fit and return the rest.
Yeah, that's the main problem with this. If you're buying higher-end stuff with tighter tolerances, just providing a set of measurements can work. There's a whole industry of tailors in India and Hong Kong who sell made-to-measure dress shirts to Westerners over the internet.
There's not a directory or anything it seems like.
I tried visiting several sites and got these results:
Works for me: Macy's, J.Crew, JCPenney
Doesn't work: Kohl's, Dockers, Lands' End, Ralph Lauren
In all cases, I picked a random (except for avoiding store brands) men's casual shirt and looked on the details page near where you select a size.
True Fit partners with several stores. You go on a store's web site (for example, macys.com), and you tell True Fit about some clothing you already have that fits well (brand, size, style). Based on that, it refers to its database to recommend the best size of the product you're considering now, and it tells you whether it thinks it will be "snug", "true to you", or "loose".
Basically it just formalizes the kind of knowledge that shoppers use about how brand A tends to run small, brand B runs large, brand C is aimed at young people so its shirts are more tailored, brand D is aimed at older people and their shirts are more "generous" in the waist, etc.
Their web site calls it the "Fashion Genome™", which is a "comprehensive, normalized fashion graph".