Make it as big as you can afford, put shelves from floor to ceiling with doors so your fabric and thread are protected from fading. Put pegboard in a few places so you can hang things like scissors and rulers where you can SEE them. If the room is big enough, put your cutting table in the middle of the room, so you can walk around it. You can also put cabinets under it - more storage! Plan on lots of electrical outlets for all the sewing machines, sergers, tv, computer. Have a separate circuit for the iron if possible. While you're at it, have the carpenters (or DH) make a big ironing board top for ironing your fabric and quilt tops. Think about where you'll need lighting before the ceiling is finished. You'll want a bathroom, fridge and comfy chair, too - both one for sewing and one for just sitting!
Post pictures while you're building and when it's finished, please!