Important: Size, cut and fit
Of course, your bike jacket should fit you! But what does that mean exactly? That depends on how and when you wear it. The more active you are, the tighter the cut can be. This makes it easier to use a mid-layer and prevents your weather protection jacket from flapping around annoyingly. However, make sure that the jacket or vest is not so tight that it restricts your freedom of movement.
A special case is insulation using the onion principle: Here it is not the fibre that warms, but mainly the air between the fibres and layers of clothing. If you go for a flexible combination of warm underwear, insulating mid-layer and rain jacket, make sure the insulation layers don't get flattened.
Outerwear for cyclists is usually cut slightly longer at the back. This way your new cycling jacket (or vest) also covers the lower back as well as the area towards your backside. This is crucial if you adopt an especially athletic position on your road bike, gravel bike or mountain bike.
For the same reason, bike jackets often have slightly longer sleeves or sleeve tops. So cycling gloves and jacket sleeves are flush and don't give rain a chance.