Wait until you finish this jacket and see how the neck works and the arm holes etc.
Then...if you like that, you can use it as a guide.
What I would do is lay your finished sweater out on a piece of news print....or a large piece of brown wrapping paper, and decide how much larger youw ant to make it for a size L....and draw that on the paper. What you are doing is making a simple paper pattern of the sweater you want to knit.
Then make a swatch using the yarn and the needles you like best and any pattern in the sweater if there is any....figure out your gauge ( how many stitches to the inch....and how many rows)....measure the pattern for the large....multiply by the number of stitches to make an inch and cast on that many stitches.
As for the neck...I would look at the pattern for the XL....see how many stitches you should cast on for that.... and see how far off that number is and do the neck like that.
I don't know the pattern....don't know if the neck is cast on....added....shaped, turtle neck or what.
Linda C

Thanks, Linda. I'm anxious to finish it so I can work out a pattern. I've never adjusted a pattern before, so this willbe a new experience. I found a great book on knit jackets, most use smaller needles than 9, but most of them look more complex than this jacket. I'm beginning to feel more experienced with this project. I think I still want to make this jacket in large as I planned as it is simple and attractive, not too bulky or puffy looking.