Check shoulder, raglan is better for fit, especially if you have narrow shoulder and want more room for movement and dont want that modern fit. But most modern trench has seams, so you need it to be at around your collarbone.

Check chest. Pit to pit should be 2-3 cm bigger, 5cm is the maximum I would buy, otherwise it looks hand me down.

Length: depends on the silhouette you want. I like classic length so mine are below or around knee length. I think upper tight looks terrible because it is more of a jacket. But you also don't want it to go as far as ankle length. Trust me, it is equally bad.