I feel like this question is slightly misplaced, but i take the smallest size they offer which is a Small. Keep in mind it is japanese sized so even though I am more of a true XXS-XS a C.E Small fits me perfectly. Within its intended context, that is.
I say this is misplaced (one for lack of proper vocab lawl) but also because the cut of the garments are very weird. For example, with sweaters the arms are huge up at the armhole and biceps but have an extreme taper towards the cuffs. Shirting is slim but the tails are super long. etc. Some people wont be into this.
The easy part is that sizing/cut is the same for every basic garment throughout the seasons (sweaters, tees, l/s tees, s/s shirts, l/s shirts, etc. all fit the same way they did when they started in 2011ish). Almost every retailer has exact measurements for the garments as well, including cavempt’s own website.
hope this helps!