People who have these issues? People with muscular arms and people with fatty arms (or a combination of both).
Anyway, back to sewing....
When I want to make something that involves sleeves and I'm using a non-stretchy fabric, I always measure the sleeve pattern. I tend to measure from about 1" down from the armpit section of the sleeve. The last pattern I used had sleeves 14" across. My arms are roughly 14" around. In order to make the sleeves comfortable for putting on and taking off, I tend to add on 1" to both sides of the pattern. This should allow for the pattern to fit and for the seam (I tend to keep mine narrow).
|Measure the pattern|
|Make extensions based on your arm size.|
|Measuring "We Can Do It" style|
If you are sewing long sleeves, make sure you measure your arm "We Can Do It" style. You want to know your maximum arm flex circumference. You don't want the shirt to only fit your arms when they're at your sides because we all have to reach for something at some point, even if it's just for the coffee creamer or something silly.
When it comes to long sleeves, there are 3 important areas that you want to measure: the arm pit (about 1" lower than arm pit), the bicep (make sure you flex those muscles), and the widest part of your lower arm (some folks have Popeye arms).
When you alter the pattern, you will want to use the widest point. Use a yard stick or something long and straight to create a straight line from the armpit corner of the pattern to that wide point to the bottom of the sleeve.
When doing this, I suggest using newspaper to make your extension pieces. Just tape your pattern to newspaper, measure, and cut out the newspaper. It's cheap and lightweight, so it will fold up nicely and fit into your pattern envelope.
I hope this has been helpful - despite the awful, quick drawings I did while at work. They should get the point across.
If you have a sewing issue, please let me know. I can help you out!