Given the equipment I currently have and expect to keep (no longarm), the smallest room I could use would be 20 x 20 (I've measured), and that would be crowded with no room for growth. I suggest that you make a HUGE built in fabric closet on one side, at least 7 feet deep, so you could have two feet of built-in shelves for fabric on each side and 3 feet to walk between them. It would be as long as your room, say 20 feet.
Think in terms of functions: you need space to support a big quilt while quilting it, space to do piecing, pressing, cutting. If you do only quilting, you will need fewer functions. If you do clothing or something else also, you need more space, such as room for a big cutting table. You might need a big ironing surface. You need to store books, threads, notions, and more. You need a design wall, which can be suspended on a dowel(s) in front of book cases. You need to have a place for WIP, and if you do string quilts, a place to store those strings. In any case, where will you put your projects?
I had to take the recliner out of my sewing room and I really miss it. You might think of leaving room for a reading/tv chair.
There is a book called Dream Sewing Spaces by Black, I think. She has good ideas.