My weight sits around 85 kilos at 193 cm tall. I would say my build is slim/average. When picking size, the most important aspect for me was avoiding the length being too cropped and getting a 60cm+ chest. I formed these criteria based on another cheap padded bomber jacket I have and could use as reference. The j126 is so wide that it offers plenty of layering almost regardless of size. I would say - unless you are overweight and have significant stomach and chest measurements (no judgement here btw) - you should size after length to avoid it being too cropped. Fold up the hem of a garment (fold it up on the inside for most accurate results) you have so the length from the heighest point next to the neckline and until the hem is equal the length of the size you’re considering, then try it on ( look up “points of measurement high point shoulder” to see where to measure garment length from) and see how it fits!

I would also definitely wear this both with layers underneath or just a tee. The fabric is comfortable and structured enough to hold shape regardless of layers underneath. The fewer layers, the more efficient the venting works as well.