The purpose of a shell jacket is to be a thin outer layer offering the wearer protection from the elements. Therefore, they are highly versatile, designed to be combined with several underlayers depending on how well-insulated the wearer needs to be. Keep this in mind when purchasing a shell jacket.
If you mainly plan to wear your shell jacket in an urban environment, and not when needing several layers to survive the extreme cold, then you should choose a jacket that fits well without having to wear a thick jumper underneath. Otherwise, the jacket may feel too large and baggy most of the time you wear it.
On the other hand, if you know you may need to wear warm clothing underneath from time to time, you should of course leave room for this when selecting your size. If you need to wear several layers to stay warm when doing physical work in cold temperatures, a good tip is to never wear cotton fabrics closest to the body. Cotton absorbs moisture, so if you begin to sweat, the fabric will become wet and quickly have a cooling effect. In this case, wool or synthetic fabrics are the best choice for the layer closest to your body. Pair your shell jacket with a smart pair of jeans and well-made formal boots for an urban outdoor look. More inspiration can be found in Carl Magazine.