Hi Lou, I guess it kind of depends on how much of your boat you are planning to do, and is it a 2320, 2520....? The difficult areas to do and come out looking professional and clean are the areas that require a seam. The cabin area is pretty straight forward, because the way I did it there are only a few very short (1"-2") seams. Try to plan it out so and seams you have are as short as possible. In the photo I am attaching, you will see how I placed the seam in a very short area.
If you plan to do the v-berth, the sidewalls are very easy. The ceiling is tricky since it has a big seam all the way across it, and if not done right it will show out like a sore thumb. I hired an upholstery guy to do my lid after realizing it was more difficult than I thought it would be.
When I started my project, I wasn't planning to do the v-berth, so I tried very hard to match the original hull liner. If I had it to do over again, and knew I was gonna do the entire interior, I wouldn't be so concerned with matching the original material. I would be more concerned with finding a material that came in wider rolls to eliminate the seam on the v-berth lid. I think I would also try to find something slightly darker, to not show dirt.
To answer your question though, you will need to measure the width of you Cabin lid and ad maybe 20%. You can use the same dimension for the back wall and side walls.