20x20 might be big enough for me, I think. A skylight, lots of windows. Center island for cutting and ironing with storage underneath. Built-in cabinets for fabric, thread. Sewing machine in cabinets around the perimeter, allowing ample room for sewing on each. A separate table for serger(s) and embroidery machine. Computer, TV, radio/cd player. Book shelves for my sewing/quilting books. The largest quilting machine/frame with all the latest features. Oh, and while we're dreaming, might as well throw in the largest embroidery machine with all the latest features.
