My sewing room is about 15 x 17 and isn't big enough. I don't have room for a long arm. I have cupboards on 2 of the 17 foot walls and that makes the 15 foot wall about 11 feet. The advantage I have is there are 3 closets in the room but they are used for storage of things like suitcases and my husbands clothes since the closet in our room is full of my clothes. lol
My humble opinion is the larger your space is the more "stuff" you accumulate. You should probably go as large as you can afford. My dream room is probably as large as a small warehouse, and I would probably out grow it as soon as I built it.
Sue