Wow, what a treat. Make the room bigger than you think you'll need.
This "hobby" has a way of out-growing the space. Two of my sewing machines (a small table for one, a koala type table I can fold out for the other, a tv, recliner, ironing board, cat tree, and a 6' cabinet with lots of drawers for tools, notions etc. in our bonus room. Then in the spare room, the bookcases for what was to be a library got surround by rulers and other cutting equipment (cutting table, too) and became the "cutting room." The only guest room left in this 4 bedroom house has a closet dedicated to floor to ceiling shelving for fabric. I can remove a couple of shelves for guests to hang their clothes. We can only park one car in the garage as this summer my husband took over 1/2 of the garage for his new long arm. Whatever size one has, never seems to be enough space. My only suggestion is to put as much built in storage in your new sewing room as you can, so most everything is at hand. Convience is not part of my quilting life--ah well, we all make it work. I remember my first few quilts were done on the kitchen table, putting up and taking down at meal time. What joy when I got a cabinet for the sewing machine.