Are you planning an addition? Our addition wasn't for a sewing room. We added a den 16x 25 across the back of our house. A screen porch of 16 x 16 continues the addition. This has become where we spend most of our time.
I would start a dream notebook with ideas an inspiration. You will want to use a good scale for determining the size. Don't make the room long and skinny. Use a measurement that already comes close to that length to avoid waste of materials.
Plan your room according to your needs or zones. How much space do you need for sewing activities? What machines do you use in your sewing room? Any future plans for specific machines? What are your storage needs?
Put electrical outlets at counter height. I would do a quad outlet or even a tower plug outlet. Consider your lighting requirements and perhaps a lighting planner. There is a formula for lumens for the size of the room.
Think about the window treatments and size of each window. Larger windows costs more to decorate,
Good luck!