FWIW. I sent my medium back after speaking to the helpful guys at Howies.
I’m a 38.5 chest so right at the top edge of Medium but the sleeves felt tight around the shoulders and underarms.
The chap said they we’re designed slim and fitted as they are a more technical jacket but a size up shouldn’t be too large.
I was advised the large, compared to the medium (at 38″), was obviously bigger in the width (by 4 inches IIRC), the length is slightly longer but the sleeves are the same length. This should help if riding with something more than a light base layer. I’m not too much concerned with it flapping around as I generally ride with a CamelBak but a bit more length and less sense of restriction over the shoulders will help.
I guess I’ll find out when the replacement comes.