3 = XS
4 = S
5 = M
Is the general consensus, yes?
Knits, if it is a thick knit should be sized TTS. In the case of #18, I think people are better off sizing TTS or down, TTS if they are in between, down if they are smaller e.g. slimmer medium, go for a small. For thinner, finer knits like the Turk Cardigan, sizing down is a bad idea. The composition doesn't lend itself to much stretch IME and can end up looking tight on the body.
Coats and jackets IMO should be TTS or down one similar to thick knits. TTS also if you care to layer.
That's just my general experience, and not solely with SS but other designers. I think it is 50/50 about being honest with your size / body type, and understanding fully the way material of the clothes is composed to best hang and wear on your body. I know length wise when people point out something is too long, perhaps the intention of the design is to have it drape.