I am having one built and it's 12 x 20. I had been using a bedroom (12x14) but my longarm takes half of the room and I can bearly squeez through to the back side. My son was going to build a storage shed so I asked for him to add a room for me.....
I have already figured out on a graft paper where things are going to fit. I will have the LA in the SW corner and my sewing stations on the north wall, my new Kenmore and Serger set up and Singer 301 and Featherweight in the room handy. I will leave an older Kenmore and the Singer Treadle in the house.
All the necessary electrical cords will be installed for computer, TV, with AC and Heating...Microwave and small fridge. Oh and a day bed!
My DH is afraid he will have to make "visits" just to see me. haha
I would really recommend drawing out what is your dream room and then look at what you can afford.